Quantum dot light-emitting device

ABSTRACT

There is provided a quantum dot light-emitting device including: a light-emitting layer containing a quantum dot luminescent material; and a metal-based particle assembly layer being a layer consisting of a particle assembly including 30 or more metal-based particles separated from each other and disposed in two-dimensions, said metal-based particles having an average particle diameter in a range of 200 to 1600 nm, an average height in a range of 55 to 500 nm, and an aspect ratio, as defined by a ratio of said average particle diameter to said average height, in a range of 1 to 8, wherein said metal-based particles that compose said metal-based particle assembly layer are disposed such that an average distance between adjacent metal-based particles may be in a range of 1 to 150 nm. The quantum dot light-emitting device provides enhanced emission via the metal-based particle assembly layer and thus presents high luminous efficiency.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2012/075059 filed Sep. 28, 2012, claiming priority based onJapanese Patent Application No. 2011-219159 filed Oct. 3, 2011, thecontents of all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a quantum dot light-emitting deviceexploiting plasmon resonance of a metal-based particle assembly inaiming for enhanced emission and containing a quantum dot luminescentmaterial in a light-emitting layer.

BACKGROUND ART

It has conventionally been known that making metal particles small to benano-sized presents functions that are not observed when it is in a bulkstate, and “localized plasmon resonance” is in particular expected forapplication. Plasmon is a compressional wave of free electrons thatarises by collective oscillation of the free electrons in a metallicnanostructure.

In recent years, a field of art handling the plasmon is referred to as“plasmonics” and attracts large attention, and has also been activelystudied and such study includes exploiting phenomena of localizedplasmon resonance of a metal nanoparticle to be intended forimprovements of light-emitting devices in luminous efficiency.

For example, Japanese Patent Laying-Open No. 2007-139540 (PTD 1)discloses a technique exploiting localized plasmon resonance forenhanced fluorescence of fluorescent substance, and Japanese PatentLaying-Open No. 2010-238775 (PTD 2) discloses an electroluminescencedevice (EL device) in which core shell type fine particles, eachcomposed of a fine metal particle core and an insulator shell coveringthe core, capable of inducing localized plasmon are arranged in avicinity of or inside a light emitting region. Furthermore, T. Fukuuraand M. Kawasaki, “Long Range Enhancement of Molecular Fluorescence byClosely Packed Submicro-scale Ag Islands”, e-Journal of Surface Scienceand Nanotechnology, 2009, 7, 653 (NPD 1) indicates a study on localizedplasmon resonance of silver nanoparticles.

On the other hand, a “quantum dot” which may exhibit a significantlyhigh quantum efficiency in principle attracts attention in recent yearsas a luminescent material for an EL device (for example, Japanese PatentLaying-Open No. 2008-214363 (PTD 3)).

CITATION LIST Patent Document

-   PTD 1: Japanese Patent Laying-Open No. 2007-139540-   PTD 2: Japanese Patent Laying-Open No. 2010-238775-   PTD 3: Japanese Patent Laying-Open No. 2008-214363

Non Patent Document

-   NPD 1: T. Fukuura and M. Kawasaki, “Long Range Enhancement of    Molecular Fluorescence by Closely Packed Submicro-scale Ag Islands”,    e-Journal of Surface Science and Nanotechnology, 2009, 7, 653

SUMMARY OF INVENTION Technical Problem

A “quantum dot” is a luminous nanoparticle with a particle diameter ofapproximately several nm to 20 nm, which forms a three-dimensionalquantum well structure and is composed of approximately several hundredsto several thousands of semiconductor atoms. For example, aconventionally known quantum dot is composed of a semiconductor particle(core) exhibiting relatively small bandgap energy and a covering layer(shell) covering a surface of the core and exhibiting relatively largebandgap energy. A quantum dot is more advantageous than conventionalluminescent materials in that, in addition to a potential of exhibitingan significantly high quantum efficiency, emission of light having adesired emission wavelength can be provided by only adjusting a particlediameter of the quantum dot.

However, a photoluminescence quantum efficiency of a quantum dot remainsrelatively low in general because of the reason such as a technicaldifficulty in manufacture of nanoparticles having even particlediameters, thus a quantum dot light-emitting device exhibiting enoughhigh luminous efficiency has not been developed.

Therefore, an object of the present invention is to provide a quantumdot light-emitting device containing a novel plasmonic material havingan enhanced emission ability to provide enhanced emission and therebypresent high luminous efficiency even when a quantum dot luminescentmaterial exhibiting a relatively low quantum efficiency is used.

Solution to Problem

PTD 1 (see paragraphs [0010] to [0011]) provides a theoreticalexplanation of a relationship between emission enhancement throughlocalized plasmon resonance and a metal nanoparticle's particlediameter, and according to this explanation, when a spherical silverparticle having a particle diameter of approximately 500 nm is used,while luminous efficiency φ of approximately 1 is theoreticallyprovided, in reality such a silver particle does not present asubstantial effect to enhance emission. Such a large-size silverparticle does not present a substantial effect to enhance emissionbecause it is inferred that the silver particle has an excessively largenumber of surface free electrons therein, and accordingly, dipole-typelocalized plasmon observed in a typical nanoparticle (a nanoparticlehaving a relatively small particle diameter) is not easily generated. Itis believed, however, that if a significantly large number of surfacefree electrons that the large-size nanoparticle has therein can beeffectively excited as plasmon, it would be expected to contribute todrastically more effective enhancement via the plasmon.

As a result of a diligent study, the present inventor has found thatwhen a large-sized metal-based particle generally believed to provide asmall emission enhancement effect, as set forth above, is formed to havea specific shape and at least a specific number of such particles aremutually separated in two dimensions and thus disposed to form ametal-based particle assembly, the assembly surprisingly can not onlypresent significantly intense plasmon resonance but also allows theplasmon resonance to have an effect over a significantly extended range(or a plasmonic enhancement effect to cover the range) and that a layer(or film) that is formed of the metal-based particle assembly isintroduced into a quantum dot light-emitting device, whereby luminousefficiency can be drastically improved.

More specifically, the present invention includes the following:

[1] A quantum dot light-emitting device comprising:

a light-emitting layer containing a quantum dot luminescent material;and

a metal-based particle assembly layer being a layer consisting of aparticle assembly including 30 or more metal-based particles separatedfrom each other and disposed in two-dimensions, said metal-basedparticles having an average particle diameter in a range of from 200 to1600 nm, an average height in a range of from 55 to 500 nm, and anaspect ratio, as defined by a ratio of said average particle diameter tosaid average height, in a range of from 1 to 8, wherein

said metal-based particles that compose said metal-based particleassembly layer are disposed such that an average distance betweenadjacent metal-based particles may be in a range of from 1 to 150 nm.

[2] A quantum dot light-emitting device comprising:

a light-emitting layer containing a quantum dot luminescent material;and

a metal-based particle assembly layer being a layer consisting of aparticle assembly including 30 or more metal-based particles separatedfrom each other and disposed in two dimensions, said metal-basedparticles having an average particle diameter in a range of from 200 to1600 nm, an average height in a range of from 55 to 500 nm, and anaspect ratio, as defined by a ratio of said average particle diameter tosaid average height, in a range of from 1 to 8, wherein

said metal-based particle assembly layer has in an absorption spectrumfor a visible light region a maximum wavelength of a peak at a longestside in wavelength, and the maximum wavelength shifts toward a shorterside in wavelength in a range of from 30 to 500 nm as compared with thatof a reference metal-based particle assembly (X) in which metal-basedparticles having a particle diameter equal to said average particlediameter and a height equal to said average height and made of the samematerial are disposed such that each distance between adjacentmetal-based particles may be in a range of from 1 to 2 μm.

[3] A quantum dot light-emitting device comprising:

a light-emitting layer containing a quantum dot luminescent material;and

a metal-based particle assembly layer being a layer consisting of aparticle assembly including 30 or more metal-based particles separatedfrom each other and disposed in two dimensions, said metal-basedparticles having an average particle diameter in a range of from 200 to1600 nm, an average height in a range of from 55 to 500 nm, and anaspect ratio, as defined by a ratio of said average particle diameter tosaid average height, in a range of from 1 to 8, wherein

said metal-based particle assembly layer has in an absorption spectrumfor a visible light region a maximum wavelength of a peak at a longestside in wavelength, and an absorbance at the maximum wavelength ishigher as compared with that of a reference metal-based particleassembly (Y) in which metal-based particles having a particle diameterequal to said average particle diameter and a height equal to saidaverage height and made of the same material are disposed such that eachdistance between adjacent metal-based particles may be in a range offrom 1 to 2 μm, on the premise that the numbers of the metal-basedparticles are the same.

[4] The quantum dot light-emitting device according to any one of items[1] to [3], wherein said metal-based particles that compose saidmetal-based particle assembly layer are oblate particles with saidaspect ratio of more than 1.

[5] The quantum dot light-emitting device according to any one of items[1] to [4], wherein said metal-based particles that compose saidmetal-based particle assembly layer are made of silver.

[6] The quantum dot light-emitting device according to any one of items[1] to [5], wherein said metal-based particles that compose saidmetal-based particle assembly layer are non-conductive between adjacentmetal-based particles.

[7] The quantum dot light-emitting device according to any one of items[1] to [6], wherein said metal-based particle assembly layer has in anabsorption spectrum for a visible light region a maximum wavelength of apeak at a longest side in wavelength, and the maximum wavelength is in arange of from 350 to 550 nm.

[8] The quantum dot light-emitting device according to any one of items[1] to [7], wherein said metal-based particle assembly layer has in anabsorption spectrum for a visible light region a maximum wavelength of apeak at a longest side in wavelength, and an absorbance at the maximumwavelength is at least 1.

[9] The quantum dot light-emitting device according to any one of items[1] to [8], further comprising an insulating layer interposed betweensaid light-emitting layer and said metal-based particle assembly layer.

[10] The quantum dot light-emitting device according to item [9],wherein said insulating layer is formed so as to cover a surface of eachmetal-based particle that composes said metal-based particle assemblylayer.

[11] The quantum dot light-emitting device according to any on of items[1] to [10], wherein a distance between a light-emitting layer sidesurface of said metal-based particle assembly layer and saidlight-emitting layer is at least 10 nm.

[12] The quantum dot light-emitting device according to any one of items[1] to [11], wherein a distance between a light-emitting layer sidesurface of said metal-based particle assembly layer and saidlight-emitting layer is at least 10 nm, and said quantum dot luminescentmaterial contained in said light-emitting layer has a photoluminescencequantum efficiency of 1.5 times or larger than that of a referencequantum dot light-emitting device that does not have said metal-basedparticle assembly layer.

[13] A method of enhancing emission of a quantum dot light-emittingdevice, the method comprising disposing in said quantum dotlight-emitting device a metal-based particle assembly layer being alayer consisting of a particle assembly including 30 or more metal-basedparticles separated from each other and disposed in two dimensions, saidmetal-based particles having an average particle diameter in a range offrom 200 to 1600 nm, an average height in a range of from 55 to 500 nm,and an aspect ratio, as defined by a ratio of said average particlediameter to said average height, in a range of from 1 to 8, wherein

said metal-based particles that compose said metal-based particleassembly layer are disposed such that an average distance betweenadjacent metal-based particles may be in a range of from 1 to 150 nm.

[14] A method of enhancing emission of a quantum dot light-emittingdevice, the method comprising disposing in said quantum dotlight-emitting device a metal-based particle assembly layer being alayer consisting of a particle assembly including 30 or more metal-basedparticles separated from each other and disposed in two dimensions, saidmetal-based particles having an average particle diameter in a range offrom 200 to 1600 nm, an average height in a range of from 55 to 500 nm,and an aspect ratio, as defined by a ratio of said average particlediameter to said average height, in a range of from 1 to 8, wherein

said metal-based particle assembly layer has in an absorption spectrumfor a visible light region a maximum wavelength of a peak at a longestside in wavelength, and the maximum wavelength shifts toward a shorterside in wavelength in a range of from 30 to 500 nm as compared with thatof a reference metal-based particle assembly (X) in which metal-basedparticles having a particle diameter equal to said average particlediameter and a height equal to said average height and made of the samematerial are disposed such that each distance between adjacentmetal-based particles may be in a range of from 1 to 2 μm.

[15] A method of enhancing emission of a quantum dot light-emittingdevice, the method comprising disposing in said quantum dotlight-emitting device a metal-based particle assembly layer being alayer consisting of a particle assembly including 30 or more metal-basedparticles separated from each other and disposed in two dimensions, saidmetal-based particles having an average particle diameter in a range offrom 200 to 1600 nm, an average height in a range of from 55 to 500 nm,and an aspect ratio, as defined by a ratio of said average particlediameter to said average height, in a range of from 1 to 8, wherein

said metal-based particle assembly layer has in an absorption spectrumfor a visible light region a maximum wavelength of a peak at a longestside in wavelength, and an absorbance at the maximum wavelength ishigher as compared with that of a reference metal-based particleassembly (Y) in which metal-based particles have a particle diameterequal to said average particle diameter and a height equal to saidaverage height and made of the same material are disposed such that eachdistance between adjacent metal-based particles may be in a range offrom 1 to 2 μm, on the premise that the numbers of the metal-basedparticles are the same.

In the present invention, the quantum dot light-emitting device is alight-emitting device containing a quantum dot luminescent material asat least a part of a luminescent material.

Advantageous Effects of Invention

According to the quantum dot-emitting device including a predeterminedmetal-based particle assembly layer as an emission-enhancing element,both emission enhancement and improvement of light extraction efficiencycan be achieved to exhibit high luminous efficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross section view of an example for a quantum dotlight-emitting device of the present invention.

FIG. 2 is a schematic cross section view of another example for aquantum dot light-emitting device of the present invention.

FIG. 3 is SEM images (as scaled 10000 times and 50000 times) of ametal-based particle assembly layer in a metal-based particle assemblylayer-layered substrate obtained in Production Example 1, as observedfrom directly above.

FIG. 4 is an AFM image of the metal-based particle assembly layer in themetal-based particle assembly layer-layered substrate obtained inProduction Example 1.

FIG. 5 is SEM images (as scaled 10000 times and 50000 times) of ametal-based particle assembly layer in a metal-based particle assemblylayer-layered substrate obtained in Production Example 2, as observedfrom directly above.

FIG. 6 is an AFM image of the metal-based particle assembly layer in themetal-based particle assembly layer-layered substrate obtained inProduction Example 2.

FIG. 7 is an SEM image (as scaled 10000 times) of a metal-based particleassembly layer in a metal-based particle assembly layer-layeredsubstrate obtained in Comparative Production Example 3, as observed fromdirectly above.

FIG. 8 is an AFM image of the metal-based particle assembly layer in themetal-based particle assembly layer-layered obtained in ComparativeProduction Example 3.

FIG. 9 is absorption spectra of the metal-based particle assemblylayer-layered substrates obtained in Production Example 1 andComparative Production Examples 1 and 2.

FIG. 10 is an absorption spectrum of the metal-based particle assemblylayer-layered substrate obtained in Production Example 2.

FIG. 11 is a schematic flow diagram showing a method for producing areference metal-based particle assembly.

FIG. 12 is SEM images (as scaled 20000 times and 50000 times) of areference metal-based particle assembly layer in a reference metal-basedparticle assembly layer-layered substrate, as observed from directlyabove.

FIG. 13 illustrates an absorption spectrum measurement method using anobjective lens (100 times) of a microscope.

FIG. 14 is an absorption spectrum of the metal-based particle assemblylayer-layered substrate obtained in Production Example 1, as measured bya method using an objective lens (100 times) of a microscope.

FIG. 15 is an absorption spectrum of the metal-based particle assemblylayer-layered substrate obtained in Production Example 2, as measured bya method using an objective lens (100 times) of a microscope.

FIG. 16 is an absorption spectrum of the metal-based particle assemblylayer-layered substrate obtained in Comparative Production Example 3, asmeasured by a method using an objective lens (100 times) of amicroscope.

FIG. 17( a) schematically shows a system to measure an emission spectrumof a quantum dot light-emitting device, and FIG. 17( b) is a schematiccross section view of a quantum dot light-emitting device subjected tomeasurement.

FIG. 18 is a graph comparing emission enhancement effects in the quantumdot light-emitting devices of Examples 2 to 5 with emission enhancementeffects in the quantum dot light-emitting devices of ComparativeExamples 2 to 6.

DESCRIPTION OF EMBODIMENTS

The present invention provides a quantum dot light-emitting deviceconfigured to include at least: a light-emitting layer containing aquantum dot luminescent material; and a metal-based particle assemblylayer that is a layer (or film) disposed in the quantum dotlight-emitting device and consists of a particle assembly including 30or more metal-based particles mutually separated and disposed in twodimensions.

In the present invention the metal-based particles that compose themetal-based particle assembly layer have an average particle diameter ina range of from 200 to 1600 nm, an average height in a range of from 55to 500 nm, and an aspect ratio, as defined by a ratio of the averageparticle diameter to the average height, in a range of from 1 to 8.

<Metal-Based Particle Assembly Layer>

The quantum dot light-emitting device of the present invention in apreferable embodiment includes a metal-based particle assembly layerhaving any of the following features:

[i] the metal-based particles that compose the metal-based particleassembly layer are disposed such that an average distance betweenadjacent metal-based particles may be in a range of from 1 to 150 nm (afirst embodiment);

[ii] the metal-based particle assembly layer has in an absorptionspectrum for a visible light region a maximum wavelength of a peak at alongest side in wavelength, and the maximum wavelength shifts toward ashorter side in wavelength in a range of from 30 to 500 nm as comparedwith that of a reference metal-based particle assembly (X) in whichmetal-based particles having a particle diameter equal to the averageparticle diameter and a height equal to the average height and made ofthe same material are disposed such that each distance between adjacentmetal-based particles may be in a range of from 1 to 2 μm (a secondembodiment); and

[iii] the metal-based particle assembly layer has in an absorptionspectrum for a visible light region a maximum wavelength of a peak at alongest side in wavelength, and an absorbance at the maximum wavelengthis higher as compared with that of a reference metal-based particleassembly (Y) in which metal-based particles having a particle diameterequal to the average particle diameter and a height equal to the averageheight and made of the same material are disposed such that eachdistance between adjacent metal-based particles may be in a range offrom 1 to 2 μm, on the premise that the numbers of the metal-basedparticles are the same (a third embodiment).

In the present specification, a metal-based particle assembly having anaverage particle diameter and an average height equal to those ofreference metal-based particle assembly (X) or (Y) means that theaverage particle diameters have a difference within a range of ±5 nm andthe average heights have a difference within a range of ±10 nm.

First Embodiment

A quantum dot light-emitting device of the present embodiment includinga metal-based particle assembly layer having the feature indicated aboveat item [i] is significantly advantageous as follows:

(1) The metal-based particle assembly layer according to the presentembodiment exhibits significantly intense plasmon resonance and thusallows a stronger emission enhancement effect than a conventionalplasmonic material, and hence drastically increased luminous efficiency.The metal-based particle assembly layer according to the presentembodiment exhibits plasmon resonance having an intensity that is not asimple sum total of localized plasmon resonances that individualmetal-based particles exhibit for a specific wavelength; rather, itexhibits an intensity larger than that. More specifically, 30 or moremetal-based particles each having a prescribed shape are spaced asprescribed, as described above, to be closely disposed, therebyindividually interacting with each other to exhibit significantlyintense plasmon resonance. This is believed to be exhibited as themetal-based particles' localized plasmons interact with each other.

Generally, when a plasmonic material is subjected to absorption spectrummeasurement through absorptiometry, a plasmon resonance peak(hereinafter also referred to as a plasmon peak) is observed as a peakin an ultraviolet to visible light region, and the plasmon peak'sabsorbance value in magnitude at a maximum wavelength thereof can beused to easily evaluate the plasmonic material's plasmon resonance inintensity, and when the metal-based particle assembly layer according tothe present embodiment that is layered on a glass substrate is subjectedto absorption spectrum measurement, it can present for a visible lightregion a maximum wavelength of a plasmon peak at a longest side inwavelength, and an absorbance at the maximum wavelength can be 1 orlarger, further 1.5 or larger, and still further approximately 2.

The metal-based particle assembly layer's absorption spectrum ismeasured through absorptiometry with the layer layered on a glasssubstrate. More specifically, the absorption spectrum is obtained asfollows: the glass substrate with the metal-based particle assemblylayer layered thereon is exposed to light of the ultraviolet to visiblelight region incident on a back surface thereof (i.e., a side oppositeto the metal-based particle assembly layer) in a direction perpendicularto a substrate surface and intensity I of transmitted lightomnidirectionally transmitted toward the metal-based particle assemblylayer is measured with an integrating sphere spectrophotometer. On theother hand, a substrate which does not have a metal-based particleassembly film and has the same thickness and the same material as thesubstrate of said metal-based particle assembly film-layered substrateis exposed at a surface thereof to the same incident light as above in adirection perpendicular to that surface and intensity I₀ of transmittedlight omnidirectionally transmitted through a side opposite to theincident surface is measured with the integrating spherespectrophotometer. Then, the absorption spectrum's axis of ordinate, orabsorbance, is expressed by the following expression:Absorbance=−log₁₀(I/I ₀)

(2) The metal-based particle assembly layer presents plasmon resonancehaving an effect over a significantly extended range (or a plasmonicenhancement effect covering the range) and thus allows a strongeremission enhancement effect than a conventional plasmonic material, andthis, as well as the above, contributes to drastically improved luminousefficiency. The plasmon resonance having an effect over thesignificantly extended range allows a light-emitting layer having alarge thickness to be entirely enhanced at the same time, therebysignificantly improving luminous efficiency of the quantum dotlight-emitting device.

Such extension effects are also believed to be exhibited as 30 or moremetal-based particles each having a prescribed shape, which are spaced,as prescribed to be closely disposed cause localized plasmonsinteracting with each other. According to the metal-based particleassembly layer of the present embodiment, the range of an effect ofplasmon resonance can be extended to approximately several hundreds nm.

Thus, the quantum dot light-emitting device of the present embodimentcan achieve effective enhancement via plasmon resonance with themetal-based particle assembly layer disposed at a position for example10 nm, further several tens nm (e.g., over 20 nm, 30 nm, or 40 nm),still further several hundreds nm away from the light-emitting layer.This means that the plasmonic material, or the metal-based particleassembly layer, can be disposed closer to a light extraction face thanthe light-emitting layer, and, furthermore, in a vicinity of a lightextraction face considerably remote from the light-emitting layer, andsignificantly efficient extraction of light can thus be achieved. Thisalso contributes to drastic enhancement of luminous efficiency.

While the metal-based particle assembly layer according to the presentembodiment employs a metal-based particle of a relatively large sizethat would alone be less prone to generate dipole-type localized plasmonfor a visible light region, the layer has at least a specific number ofsuch metal-based particles of a large size (which are each required tohave a prescribed shape) spaced as prescribed to be closely disposed sothat a significantly large number of surface free electrons that thelarge-sized metal-based particles include therein can be effectivelyexcited as plasmon to achieve significantly intense plasmon resonanceand plasmon resonance having an effect over a significantly extendedrange.

On the other hand, in the emission enhancement using localized plasmonresonance via conventional metal nanoparticles, a range of the emissionenhancement effect (an effective range of localized plasmon resonance)is significantly narrow so that an emission enhancement effect could beobtained at a small fraction of the light-emitting layer, thus enoughemission enhancement effect could not be obtained. For the purpose ofaddressing such a disadvantage, when metal nanoparticles are disposed ina vicinity of or inside the light-emitting layer as disclosed in PTD 2,at least a part of emitted light is reflected toward a directiondifferent from a light extraction face at an interface of each layerdisposed between metal nanoparticles and the light extraction face,thereby lowering a light extraction efficiency.

Further, as the quantum dot light-emitting device of the presentembodiment has a metal-based particle assembly layer configured suchthat at least a specific number of metal-based particles of a relativelylarge size having a specific shape are spaced in two dimensions, asprescribed, it can thereby have an advantageous effect, as follows.

(3) The metal-based particle assembly layer according to the presentembodiment can present in an absorption spectrum for a visible lightregion a plasmon peak whose maximum wavelength presents a unique shiftdepending on its metal-based particles' average particle diameter andaverage interparticle distance, and thus allows light emission of aspecific (or desired) wavelength range to be particularly enhanced. Morespecifically, when the metal-based particles have a fixed averageinterparticle distance while having increased average particlediameters, a plasmon peak at a longest side in wavelength for thevisible light region has a maximum wavelength shifting toward a shorterside in wavelength (or blue-shifted). Similarly, when the large-sizemetal-based particles have a fixed average particle diameter whilehaving decreased average interparticle distances (i.e., when theparticles are disposed more closely), a plasmonic peak at a longest sidein wavelength for the visible light region has a maximum wavelengthshifting toward a shorter side in wavelength. This unique phenomenon iscontradictory to the Mie-scattering theory generally accepted regardingplasmon materials (according to this theory, larger particle diametersresult in a plasmon peak having a maximum wavelength shifting toward alonger side in wavelength (or red-shifted)).

It is believed that the unique blue shift as described above is alsoattributed to the fact that the metal-based particle assembly layer isstructured with large-size metal-based particles spaced as prescribed tobe closely disposed, followed by the metal-based particles having theirlocalized plasmons interacting with each other. The metal-based particleassembly layer according to the present embodiment (when it is layeredon a glass substrate) can present in an absorption spectrum for avisible light region, as measured through absorptiometry, a maximumwavelength of a plasmon peak at a longest side in wavelength, and themaximum wavelength can be in a wavelength range of for example from 350to 550 nm, depending on the metal-based particles' shape andinterparticle distance. Furthermore, the metal-based particle assemblylayer according to the present embodiment can typically cause a blueshift of approximately from 30 to 500 nm (e.g., 30 to 250 nm) ascompared with that having metal-based particles with a sufficientlylarge interparticle distance (for example of 1 μm).

A metal-based particle assembly layer having a maximum wavelength of aplasmon peak blue-shifted as described above, for example, a metal-basedparticle assembly layer having a plasmon peak in a blue wavelength rangeor a range close thereto is significantly useful in enhancing emissionof a quantum dot light-emitting device emitting light in a bluewavelength range or a range close thereto for which there is a strongdemand for enhancement of luminous efficiency.

Next will be described a specific configuration of the metal-basedparticle assembly layer according to the present embodiment.

Metal-based particles that compose the metal-based particle assemblylayer are not specifically restricted as long as made of a materialhaving a plasmon peak in an ultraviolet to visible light region inabsorption spectrum measurement through absorptiometry in the form ofnanoparticles or an assembly of such particles, and the material caninclude, for example, noble metals such as gold, silver, copper,platinum and palladium; metals such as aluminum and tantalum; alloyscontaining these noble metals or these metals; and metal compoundsincluding these noble metals or these metals (such as metal oxides, andmetal salts). Inter alia, noble metals such as gold, silver, copper,platinum and palladium are preferable, and silver is more preferable asit is inexpensive and provides small absorption (or has a smallimaginary part of a dielectric function in visible light wavelengths).

The metal-based particles have an average particle diameter within arange of from 200 to 1600 nm, and to effectively obtain the effects ofitems (1) to (3) it falls within a range preferably of from 200 to 1200nm, more preferably from 250 to 500 nm, still more preferably from 300to 500 nm. It should be noted here that a metal-based particle of alarge size having an average diameter for example of 500 nm is alone notobserved to show substantially effective enhancement via localizedplasmon. In contrast, the metal-based particle assembly layer accordingto the present embodiment has at least a prescribed number of (30) suchlarge-size metal-based particles spaced as prescribed to be closelydisposed, thereby achieving significantly intense plasmon resonance andplasmon resonance having an effect over a significantly extended range,and furthermore, the effect of item (3).

The average particle diameter of the metal-based particle, as referredto herein, is obtained as follows: a metal-based particle assembly layerhaving metal-based particles disposed in two dimensions is observed withan SEM from directly above to obtain an SEM image thereof, and thereinten particles are selected at random and in each particle's image 5tangential diametrical lines are drawn at random (note that the straightlines serving as the tangential diametrical lines can pass through onlyinside the image of the particle and one of the lines is a straight linepassing through only inside the particle and drawable to be the longest)and their average value serves as the particle's diameter and the 10selected particles' respective such particle diameters are averaged toobtain the average particle diameter of the metal-based particle. Thetangential diametrical line is defined as a perpendicular lineconnecting a spacing between two parallel lines sandwiching theparticle's contour (in a projected image) in contact therewith (see theNikkan Kogyo Shimbun, Ltd., “Particle Measurement Technique”, 1994, page5).

The metal-based particle has an average height within a range of from 55to 500 nm, and to effectively obtain the effects of items (1) to (3) itfalls within a range of preferably from 55 to 300 nm, more preferablyfrom 70 to 150 nm. The average height of the metal-based particle isobtained as follows: the metal-based particle assembly layer (or film)is observed with an AFM to obtain an AFM image thereof and therein 10particles are selected at random and measured in height and theirmeasurements are averaged to obtain the average height.

The metal-based particle has an aspect ratio within a range of from 1 to8 and to effectively obtain the effects of items (1) to (3) it fallswithin a range preferably of from 2 to 8, more preferably from 2.5 to 8.The aspect ratio of the metal-based particle is defined as a ratio ofthe above average particle diameter to the above average height (i.e.,average particle diameter/average height). While the metal-basedparticle may be spherical, preferably it is oblate having an aspectratio exceeding 1.

While the metal-based particle preferably has a smoothly curved surfacein view of exciting significantly effective plasmon and it is morepreferable that the metal-based particle be oblate having a smoothlycurved surface, the metal-based particle may have a surface with smallrecesses and projections (or roughness) to some extent and in that sensethe metal-based particle may be indefinite in shape.

Preferably, the metal-based particles have variation therebetween insize as minimal as possible in view of uniformity in intensity ofplasmon resonance within a plane of the metal-based particle assemblylayer. Even if there is a small variation in particle diameter, it isnot preferable that large-size particles have an increased distancetherebetween and it is preferable that particles of small size beintroduced between the large-size particles to help the large-sizeparticles to exhibit their interaction.

The metal-based particle assembly layer according to the presentembodiment has adjacent metal-based particles disposed to have anaverage distance therebetween (average interparticle distance) within arange of from 1 to 150 nm. Such closely disposed metal-based particlescan realize significantly intense plasmon resonance and plasmonresonance having an effect over a significantly extended range, andfurthermore, the effect of item (3). The average interparticle distanceis preferably within a range of from 1 to 100 nm, more preferably from 1to 50 nm, still more preferably from 1 to 20 nm to effectively obtainthe effects of items (1) to (3). An average interparticle distancesmaller than 1 nm results in occurrence of electron transfer between theparticles attributed to the Dexter mechanism, which disadvantageouslydeactivates localized plasmon.

The average interparticle distance, as referred to herein, is obtainedas follows. A metal-based particle assembly layer having metal-basedparticles disposed in two dimensions is observed with an SEM fromdirectly above to obtain an SEM image thereof, and therein 30 particlesare selected at random and for each selected particle an interparticledistance to an adjacent particle is obtained and the 30 particles' suchinterparticle distances are averaged to obtain an average interparticledistance. In obtaining an interparticle distance to an adjacentparticle, a distance to any adjacent particle (as obtained between theirsurfaces) is measured, and such measurements are averaged to obtain theinterparticle distance.

The metal-based particle assembly layer includes 30 or more metal-basedparticles, preferably 50 or more metal-based particles. The 30 or moremetal-based particles assembled together have their localized plasmonsinteracting with each other and thus exhibit significantly intenseplasmon resonance and plasmon resonance having an effect over asignificantly extended range.

In light of a typical device area of the quantum dot light-emittingdevice, the metal-based particle assembly can include 300 or moremetal-based particles, and furthermore, 17500 or more metal-basedparticles, for example.

The metal-based particle assembly layer includes metal-based particleshaving a number density preferably of 7 particles/μm² or larger, morepreferably 15 particles/μm² or larger.

The metal-based particle assembly layer preferably has metal-basedparticles insulated from each other, that is, the layer isnon-conductive between adjacent metal-based particles (or themetal-based particle assembly layer is non-conductive). If some or allof the metal-based particles can pass/receive electrons to/from eachother, the plasmon peak loses sharpness and thus resembles an absorptionspectrum of bulk metal, and high plasmon resonance is not obtained,either. Accordingly, it is preferable that the metal-based particles besurely separated and have no conductive substance interposedtherebetween.

Second Embodiment

The present embodiment provides a quantum dot light-emitting deviceincluding a metal-based particle assembly layer showing in an absorptionspectrum for a visible light region a maximum wavelength of a peak at alongest side in wavelength, and the maximum wavelength shifts toward ashorter side in wavelength in a range of from 30 to 500 nm as comparedwith that of the reference metal-based particle assembly (X) (or havingthe feature indicated above at item [ii]). The quantum dotlight-emitting device of the present embodiment including a metal-basedparticle assembly layer having such a feature is significantlyadvantageous as follows.

(I) The metal-based particle assembly layer according to the presentembodiment shows in an absorption spectrum for the visible light regiona maximum wavelength of a plasmon peak at a longest side in wavelength,and the maximum wavelength presents in a unique wavelength range,thereby allowing light emission of a specific (or desired) wavelengthrange to be particularly enhanced. Specifically, when the metal-basedparticle assembly layer according to the present embodiment is subjectedto absorption spectrum measurement, it presents the plasmon peak with amaximum wavelength shifted to a shorter side (or blue-shifted) inwavelength in a range of from 30 to 500 nm (e.g., from 30 to 250 nm) ascompared with a maximum wavelength of reference metal-based particleassembly (X) described later, and typically the plasmon peak has themaximum wavelength within a range of from 350 to 500 nm.

It is believed that the blue shift as described above is attributed tothe fact that the metal-based particle assembly layer is structured withat least a specific number of large-size metal-based particles eachhaving a specific shape separated in two dimensions, followed by themetal-based particles having their localized plasmons interacting witheach other.

As described above, the metal-based particle assembly layer having amaximum wavelength of a plasmon peak blue-shifted, for example, themetal-based particle assembly layer having a plasmon peak in a bluewavelength range or a range close thereto is significantly useful inenhancing emission of a quantum dot light-emitting device emitting lightin a blue wavelength range or a range close thereto for which there is astrong demand for enhancement of luminous efficiency.

When a metal-based particle assembly and reference metal-based particleassembly (X) are observed to compare the maximum wavelengths of theirpeaks at a longest side in wavelength and the absorbances at the maximumwavelengths, a microscope (“OPTIPHOT-88” produced by Nikon) and aspectrophotometer (“MCPD-3000” produced by Otsuka Electronics Co., Ltd.)are used to perform absorption spectrum measurement in a narrowed fieldof view.

Reference metal-based particle assembly (X) is a metal-based particleassembly in which metal-based particles A that have a particle diameterand a height equal to the average particle diameter and the averageheight of a metal-based particle assembly layer subject to absorptionspectrum measurement and are identical in material to the metal-basedparticles of the metal-based particle assembly layer are disposed suchthat each distance between adjacent metal-based particles may be in arange of from 1 to 2 μm, and reference metal-based particle assembly (X)has a size allowing reference metal-based particle assembly (X) layeredon a glass substrate to undergo absorption spectrum measurement via amicroscope, as described above.

The wave pattern of reference metal-based particle assembly (X)'sabsorption spectrum is also theoretically calculatable in the 3D-FDTDmethod using the particle diameter and height of metal-based particle A,the dielectric function of a material of metal-based particle A, thedielectric function of a medium (e.g., air) surrounding metal-basedparticle A, and the dielectric function of the substrate (e.g., a glasssubstrate).

Furthermore, as the quantum dot light-emitting device of the presentembodiment has a metal-based particle assembly layer structured with atleast a specific number of metal-based particles of a relatively largesize having a specific shape separated in two dimensions, (II) themetal-based particle assembly layer can exhibit significantly intenseplasmon resonance and thus allows a stronger emission enhancement effectthan a conventional plasmonic material, and hence drastically increasedluminous efficiency (which is similar to effect (1) of the firstembodiment), and (III) the metal-based particle assembly layer canpresent plasmon resonance having an effect over a significantly extendedrange (or a plasmonic enhancement effect covering the range) and thusallows a stronger emission enhancement effect than a conventionalplasmonic material, and can similarly contribute to drasticallyincreased luminous efficiency (which is similar to effect (2) of thefirst embodiment), and the like. When the metal-based particle assemblylayer according to the present embodiment that is layered on a glasssubstrate is subjected to absorption spectrum measurement, it canpresent for the visible light region a maximum wavelength of a plasmonpeak at a longest side in wavelength, and an absorbance at the maximumwavelength can be 1 or larger, further 1.5 or larger, and still furtherapproximately 2.

Next will be described a specific configuration of the metal-basedparticle assembly layer according to the present embodiment. Themetal-based particle assembly layer according to the present embodimentcan have a specific configuration (for the material, average particlediameter, average height, aspect ratio, average interparticle distance,and count of the metal-based particles, the metal-based particleassembly layer's non-conductance, and the like) basically similar tothat of the metal-based particle assembly layer according to the firstembodiment. Average particle diameter, average height, aspect ratio, andaverage interparticle distance are defined as in the first embodiment.

The metal-based particles have an average particle diameter within arange of from 200 to 1600 nm, and to effectively obtain the effects ofitems (I) to (III) it falls within a range preferably of from 200 to1200 nm, more preferably from 250 to 500 nm, still more preferably from300 to 500 nm. The metal-based particle assembly layer according to thepresent embodiment is an assembly of at least a specific number of (30)large-size metal-based particles disposed in two dimensions and thus canrealize significantly intense plasmon resonance and plasmon resonancehaving an effect over a significantly extended range. Furthermore, toalso present the feature of item [ii] (i.e., a plasmon peak shifted to ashorter side in wavelength), it is essential that the metal-basedparticle has a large size with an average particle diameter of 200 nm orlarger, preferably 250 nm or larger.

The metal-based particle assembly layer according to the presentembodiment shows for a visible light region a maximum wavelength of aplasmon peak at a longest side in wavelength, and the maximum wavelengthdepends on the metal-based particles' average particle diameter. Morespecifically, when the metal-based particles have an average particlediameter exceeding a certain value, the plasmon peak has the maximumwavelength shifting toward a shorter side in wavelength (orblue-shifted).

The metal-based particles have an average height within a range of from55 to 500 nm, and to effectively obtain the effects of items (I) to(III) it falls within a range preferably of from 55 to 300 nm, morepreferably from 70 to 150 nm. The metal-based particle has an aspectratio within a range of from 1 to 8 and to effectively obtain theeffects of items (I) to (III) it falls within a range preferably of from2 to 8, more preferably from 2.5 to 8. While the metal-based particlemay be spherical, preferably it is oblate having an aspect ratioexceeding 1.

While the metal-based particle preferably has a smoothly curved surfacein view of exciting significantly effective plasmon and in particular itis more preferable that the metal-based particle be oblate having asmoothly curved surface, the metal-based particle may have a surfacewith small recesses and projections (or roughness) to some extent and inthat sense the metal-based particle may be indefinite in shape.Preferably, the metal-based particles have variation therebetween insize as minimal as possible in view of uniformity in intensity ofplasmon resonance within a plane of the metal-based particle assemblylayer. Note, however, that, as has been set forth above, even if thereis a small variation in particle diameter, it is not preferable thatlarge-size particles have an increased distance therebetween, but it ispreferable that particles of small size be introduced between thelarge-size particles to help the large-size particles to exhibit theirinteraction.

Preferably, the metal-based particle assembly layer according to thepresent embodiment has adjacent metal-based particles disposed to havean average distance therebetween (or an average interparticle distance)within a range of from 1 to 150 nm. More preferably, it is within arange of from 1 to 100 nm, still more preferably from 1 to 50 nm,particularly more preferably from 1 to 20 nm. Such closely disposedmetal-based particles present the metal-based particles' localizedplasmons interacting with each other effectively and thus facilitatepresenting the effects of items (I) to (III). As a maximum wavelength ofthe plasmon peak depends on the metal-based particles' averageinterparticle distance, the average interparticle distance can beadjusted to control to what extent a plasmon peak at a longest side inwavelength should be blue-shifted, and the plasmon peak's maximumwavelength. An average interparticle distance smaller than 1 nm resultsin occurrence of electron transfer between the particles attributed tothe Dexter mechanism, which disadvantageously deactivates localizedplasmon.

Another means other than the above means to present the feature of item[ii] (i.e., a plasmon peak shifted to a shorter side in wavelength) canfor example be introducing between the metal-based particles adielectric substance having a dielectric constant different from that ofair (which is preferably a non-conductive substance, as will bedescribed later).

The metal-based particle assembly layer includes 30 or more metal-basedparticles, preferably 50 or more metal-based particles. 30 or moremetal-based particles assembled together present the metal-basedparticles' localized plasmons interacting with each other effectivelyand thus allow the feature of item [ii] and the effects of items (I) to(III) to be presented.

In light of a typical device area of the quantum dot light-emittingdevice, the metal-based particle assembly can include 300 or moremetal-based particles, and furthermore, 17500 or more metal-basedparticles, for example.

The metal-based particle assembly layer includes metal-based particleshaving a number density preferably of 7 particles/μm² or larger, morepreferably 15 particles/μm² or larger.

The metal-based particle assembly layer according to the presentembodiment, as well as that of the first embodiment, preferably hasmetal-based particles insulated from each other, that is, the layer isnon-conductive between adjacent metal-based particles (or themetal-based particle assembly layer is non-conductive).

Third Embodiment

The present embodiment provides a quantum dot light-emitting deviceincluding a metal-based particle assembly layer showing in an absorptionspectrum for a visible light region a maximum wavelength of a peak at alongest side in wavelength, and an absorbance at the maximum wavelengthis higher as compared with that of reference metal-based particleassembly (Y) (or having the feature indicated above at item [iii]), onthe premise that the numbers of metal-based particles are the same. Thequantum dot light-emitting device of the present embodiment including ametal-based particle assembly layer having such a feature issignificantly advantageous in the following points.

(A) The metal-based particle assembly layer according to the presentembodiment shows for a visible light region a maximum wavelength of apeak at a longest side in wavelength, or a plasmon peak, and anabsorbance at the maximum wavelength is higher as compared with that ofreference metal-based particle assembly (Y) that can be regarded as anassembly of metal-based particles simply assembled together without anyinterparticle interaction. Accordingly, the metal-based particleassembly layer exhibits significantly intense plasmon resonance, andthus allows a stronger emission enhancement effect than a conventionalplasmonic material, and hence drastically increased luminous efficiency.It is believed that such intense plasmon resonance is exhibited as themetal-based particles present localized plasmons interacting with eachother.

As has been described above, the plasmon peak's absorbance value inmagnitude at the maximum wavelength thereof can be used to easilyevaluate the plasmonic material's plasmon resonance in intensity, andwhen the metal-based particle assembly layer according to the presentembodiment that is layered on a glass substrate is subjected toabsorption spectrum measurement, it can present for a visible lightregion a maximum wavelength of a plasmon peak at a longest side inwavelength, and an absorbance at the maximum wavelength can be 1 orlarger, further 1.5 or larger, and still further approximately 2.

As has been previously described, when a metal-based particle assemblyand reference metal-based particle assembly (Y) are observed to comparethe maximum wavelengths of their peaks at a longest side in wavelengthand the absorbances at the maximum wavelengths, a microscope(“OPTIPHOT-88” produced by Nikon) and a spectrophotometer (“MCPD-3000”produced by Otsuka Electronics Co., Ltd.) are used to perform absorptionspectrum measurement in a narrowed field of view.

Reference metal-based particle assembly (Y) is a metal-based particleassembly in which metal-based particles B that have a particle diameterand a height equal to the average particle diameter and average heightof a metal-based particle assembly layer subject to absorption spectrummeasurement and are identical in material to the metal-based particlesof the metal-based particle assembly layer are disposed such that eachdistance between adjacent metal-based particles may be in a range offrom 1 to 2 μm, and reference metal-based particle assembly (Y) has asize allowing reference metal-based particle assembly (Y) layered on aglass substrate to undergo absorption spectrum measurement via amicroscope, as described above.

When the metal-based particle assembly layer subject to absorptionspectrum measurement and reference metal-based particle assembly (Y) arecompared in their absorbances at the maximum wavelengths of their peaksat a longest side in wavelength, an absorption spectrum of referencemetal-based particle assembly (Y) as converted to have the same numberof metal-based particles is obtained and an absorbance at a maximumwavelength of a peak in that absorption spectrum, which peak is presentat a longest side in wavelength, is used as a target for comparison, aswill be described hereinafter. Specifically, an absorption spectrum ofthe metal-based particle assembly and that of reference metal-basedparticle assembly (Y) are obtained and the absorbances at the maximumwavelengths of the peaks in the absorption spectra, which peaks arepresent at a longest side in wavelength, respectively, are divided bytheir respective coverages (i.e., the coverages of their respectivesubstrates' surfaces by the metal-based particles), and the obtainedvalues are compared.

Furthermore, as the quantum dot light-emitting device of the presentembodiment has a metal-based particle assembly layer structured with atleast a specific number of metal-based particles of a relatively largesize having a specific shape separated in two dimensions, it can havesuch effects as follows: (B) the metal-based particle assembly layerpresents plasmon resonance that can have an effect over a significantlyextended range (or a plasmonic enhancement effect that can cover therange) and the layer thus allows a stronger emission enhancement effectthan a conventional plasmonic material, and hence drastically increasedluminous efficiency (as well as effect (2) of the first embodiment); (C)the metal-based particle assembly layer can exhibit a plasmon peakhaving a maximum wavelength uniquely shifted and thus allows emission ofa specific (or desired) wavelength range to be particularly enhanced (aswell as effect (3) of the first embodiment); and the like.

The metal-based particle assembly layer of the present embodiment (whenit is layered on a glass substrate) can present in an absorptionspectrum for a visible light region, as measured through absorptiometry,a maximum wavelength of a plasmon peak at a longest side in wavelength,and the maximum wavelength can be in a range of for example from 350 to550 nm, depending on the metal-based particles' shape and interparticledistance. Furthermore, the metal-based particle assembly layer of thepresent embodiment can typically cause a blue shift of approximatelyfrom 30 to 500 nm (e.g., from 30 to 250 nm) as compared with that havingmetal-based particles with a sufficiently large interparticle distance(for example of 1 μm).

Next will be described a specific configuration of the metal-basedparticle assembly layer according to the present embodiment. Themetal-based particle assembly layer according to the present embodimentcan have a specific configuration (for the material, average particlediameter, average height, aspect ratio, average interparticle distance,and count of the metal-based particles, the metal-based particleassembly layer's non-conductance, and the like) basically similar tothat of the metal-based particle assembly layer according to the firstembodiment. Average particle diameter, average height, aspect ratio, andaverage interparticle distance are defined as in the first embodiment.

The metal-based particles have an average particle diameter within arange of from 200 to 1600 nm, and to effectively obtain the feature ofitem [iii] (i.e., to have an absorbance at a maximum wavelength of aplasmon peak at a longest side in wavelength, which absorbance is largerthan that of reference metal-based particle assembly (Y)), andfurthermore the effects of items (A) to (C), it falls within a rangepreferably of from 200 to 1200 nm, more preferably from 250 to 500 nm,still more preferably from 300 to 500 nm. Thus it is important to userelatively large-size metal-based particles, and at least a specificnumber of (30) such large-size metal-based particles disposed in twodimensions and thus assembled together can achieve significantly intenseplasmon resonance and furthermore, plasmon resonance having an effectover a significantly extended range and a plasmon peak shifted to ashorter side in wavelength.

The metal-based particles have an average height within a range of from55 to 500 nm, and to effectively obtain the feature of item [iii] andfurthermore, the effects of items (A) to (C), it falls within a rangepreferably of from 55 to 300 nm, more preferably from 70 to 150 nm. Themetal-based particle has an aspect ratio within a range of from 1 to 8and to effectively obtain the feature of item [iii] and furthermore, theeffects of items (A) to (C) it falls within a range preferably of from 2to 8, more preferably from 2.5 to 8. While the metal-based particle maybe spherical, preferably it is oblate having an aspect ratio exceeding1.

While the metal-based particle preferably has a smoothly curved surfacein view of exciting significantly effective plasmon and in particular itis more preferable that the metal-based particle be oblate having asmoothly curved surface, the metal-based particle may have a surfacewith small recesses and projections (or roughness) to some extent and inthat sense the metal-based particle may be indefinite in shape.

Preferably, the metal-based particle assembly layer is configured of asuniform metal-based particles as possible in size and shape (averageparticle diameter, average height, and aspect ratio), as suchmetal-based particles can effectively achieve the feature of item [iii].More specifically, uniforming the metal-based particles in size andshape provides a sharp plasmon peak, followed by that an absorbance of aplasmon peak at a longest side in wavelength facilitates being higherthan that of reference metal-based particle assembly (Y). Metal-basedparticles less varying in size and shape are also advantageous in viewof uniformity in intensity of plasmon resonance within a plane of themetal-based particle assembly layer. Note, however, that, as has beenset forth above, even if there is a small variation in particlediameter, it is not preferable that large-size particles have anincreased distance therebetween, but it is preferable that particles ofsmall size be introduced between the large-size particles to help thelarge-size particles to exhibit their interaction.

Preferably, the metal-based particle assembly layer according to thepresent embodiment has adjacent metal-based particles disposed to havean average distance therebetween (or an average interparticle distance)within a range of from 1 to 150 nm. More preferably, it is within arange of from 1 to 100 nm, still more preferably from 1 to 50 nm,particularly more preferably from 1 to 20 nm. Such closely disposedmetal-based particles present the metal-based particles' localizedplasmons interacting with each other effectively and thus allow thefeature of item [iii] and furthermore, the effects of items (A) to (C)to be effectively presented. An average interparticle distance smallerthan 1 nm results in occurrence of electron transfer between theparticles attributed to the Dexter mechanism, which disadvantageouslydeactivates localized plasmon.

The metal-based particle assembly layer includes 30 or more metal-basedparticles, preferably 50 or more metal-based particles. 30 or moremetal-based particles assembled together present the metal-basedparticles' localized plasmons interacting with each other effectivelyand thus allow the feature of item [iii] and furthermore, the effects ofitems (A) to (C) to be effectively presented.

In light of a typical device area of the quantum dot light-emittingdevice, the metal-based particle assembly can include 300 or moremetal-based particles, and furthermore, 17500 or more metal-basedparticles, for example.

The metal-based particle assembly layer includes metal-based particleshaving a number density preferably of 7 particles/μm² or larger, morepreferably 15 particles/μm² or larger.

The metal-based particle assembly layer according to the presentembodiment, as well as that of the first embodiment, preferably hasmetal-based particles insulated from each other, that is, the layer isnon-conductive between adjacent metal-based particles (or the layer isnon-conductive).

Thus the metal-based particle assembly layer according to the presentembodiment having the feature of item [iii] can be obtained bycontrolling its constituent metal-based particles in metal type, size,shape, interparticle distance, and the like.

The metal-based particle assembly layer that the quantum dotlight-emitting device of the present invention includes preferably hasthe feature of any one of items [i]-[iii], more preferably any two ormore thereof, and still more preferably all thereof.

<Method for Producing Metal-Based Particle Assembly Layer>

A metal-based particle assembly layer according to the present inventionincluding the metal-based particle assembly layers according to thefirst to third embodiments can be produced in such a method as follows:

(1) a bottom-up method to grow metal-based particles from minute seedson a substrate;

(2) a method in which a metal-based particle that has a prescribed shapeis covered with a protection layer made of an amphiphilic material andhaving a prescribed thickness, and the resultant is then subjected toLangmuir Blodgett (LB) deposition to be formed in a film on a substrate;and

(3) other methods, such as a method of post-treating a thin filmobtained by vapor deposition, sputtering or the like; resist-processing;etching processing; a casting method using a liquid having metal-basedparticles dispersed therein, and the like.

It is important that method (1) includes the step of growing ametal-based particle at a significantly low speed on a substrateadjusted to have a prescribed temperature (hereinafter also referred toas the particle growth step). A production method including the particlegrowth step can provide a satisfactorily controlled layer (or thin film)of a metal-based particle assembly having 30 or more metal-basedparticles mutually separated and thus disposed in two dimensions, andhaving a shape within a prescribed range (an average particle diameterof 200 to 1600 nm, an average height of 55 to 500 nm, and an aspectratio of 1 to 8) and still preferably an average interparticle distancewithin a prescribed range (from 1 to 150 nm).

In the particle growth step, the metal-based particle is grown on thesubstrate preferably at an average height growth rate smaller than 1nm/minute, more preferably 0.5 nm/minute or smaller. The average heightgrowth rate as referred to herein can also be referred to as an averagedeposition rate or the metal-based particle's average thickness growthrate, and is defined by the following expression:

metal-based particle's average height/metal-based particle growth time(supplying time of a metal-based material).

The “metal-based particle's average height” is defined as set forthabove.

In the particle growth step, the substrate is set in temperaturepreferably within a range of from 100 to 450° C., more preferably from200 to 450° C., still more preferably from 250 to 350° C., andparticularly still more preferably 300° C. or therearound (300°C.±approximately 10° C.).

When the production method includes the particle growth step to growmetal-based particles at an average height growth rate smaller than 1nm/minute on a substrate adjusted in temperature within the range offrom 100 to 450° C., the particles are initially grown such that asupplied metal-based material forms a plurality of island structures,and as the metal-based material is further supplied the islandstructures are further grown and thus merged with neighboring islandstructures, and consequently, metal-based particles having a relativelylarge average particle diameter are closely disposed while metal-basedparticles each are completely separated from each other to form ametal-based particle assembly layer. Thus, a metal-based particleassembly layer can be produced that is formed of metal-based particlescontrolled to have a shape within a prescribed range (in averageparticle diameter, average height, and aspect ratio) and stillpreferably an average interparticle distance within a prescribed range.

Furthermore, the average height growth rate, the substrate's temperatureand/or the metal-based particle growth time (the supplying time of themetal-based material) can be adjusted to also control within aprescribed range the average particle diameter, the average height, theaspect ratio, and/or the average interparticle distance of themetal-based particles grown on the substrate.

Furthermore, the production method including the particle growth stepalso allows the particle growth step to be performed such thatconditions other than the substrate's temperature and the average heightgrowth rate are selected relatively freely, and the method thus alsoadvantageously allows a metal-based particle assembly layer of a desiredsize to be efficiently formed on a substrate of a desired size.

If the average height growth rate is 1 nm/minute or larger, or thesubstrate has a temperature lower than 100° C. or higher than 450° C.,then before the island structure is grown to be large the islandstructure forms a continuum with a neighboring island structure and ametal-based assembly formed of metal-based particles mutually completelyseparated and having a large particle diameter cannot be obtained or ametal-based assembly formed of metal-based particles having a desiredshape cannot be obtained (for example, it would depart in averageheight, average interparticle distance, and aspect ratio from a desiredrange).

While the metal-based particles are grown under a pressure (in anapparatus's chamber), which may be any pressure that allows theparticles to be grown, it is normally lower than atmospheric pressure.While the pressure's lower limit is not limited to a specific value, itis preferably 6 Pa or larger, more preferably 10 Pa or larger, stillmore preferably 30 Pa or larger, as such pressure helps to adjust theaverage height growth rate within the range indicated above.

The metal-based particles can specifically be grown on a substrate inany method allowing the particles to be grown at an average heightgrowth rate smaller than 1 nm/minute, and the method can includesputtering, and vapor deposition such as vacuum deposition. Preferablesputtering is direct current (DC) sputtering as it allows a metal-basedparticle assembly layer to be grown relatively conveniently and alsofacilitates maintaining the average height growth rate smaller than 1nm/minute. The sputtering may be done in any system and it can forexample be an ion gun, or be direct current argon ion sputtering toexpose a target to argon ions generated by a plasma discharge andaccelerated in an electric field. The sputtering is done with a currentvalue, a voltage value, a substrate-to-target distance and otherconditions adjusted as appropriate to grow particles at the averageheight growth rate smaller than 1 nm/minute.

Note that to obtain a satisfactorily controlled metal-based particleassembly layer formed of metal-based particles having a shape within aprescribed range (in average particle diameter, average height, andaspect ratio) and still preferably an average interparticle distancewithin a prescribed range, it is preferable that the particle growthstep be performed at the average height growth rate smaller than 1nm/minute and in addition thereto an average particle diameter growthrate smaller than 5 nm, and when the average height growth rate issmaller than 1 nm/minute, the average particle diameter growth rate isnormally smaller than 5 nm. The average particle diameter growth rate ismore preferably 1 nm/minute or smaller. The average particle diametergrowth rate is defined by the following expression:

metal-based particle's average particle diameter/metal-based particlegrowth time (supplying time of a metal-based material).

The “metal-based particle's average particle diameter” is defined as setforth above.

The metal-based particle growth time (the supplying time of ametal-based material) in the particle growth step is a period of timethat at least allows metal-based particles carried on a substrate toattain a shape within a prescribed range and still preferably an averageinterparticle distance within a prescribed range and that is smallerthan a period of time allowing the particles to depart from the shapewithin the prescribed range and the average interparticle distancewithin the prescribed range. For example, even though particle growth isperformed at an average height growth rate and substrate temperaturewithin the above prescribed ranges, an significantly long period of timefor growth results in a metal-based material carried in an excessivelylarge amount and accordingly it will not form an assembly of mutuallyseparated metal-based particles and instead form a continuous film or bemetal-based particles excessively large in average particle size oraverage height.

Accordingly it is necessary to grow metal-based particles for anappropriately set period of time (or to stop the particle growth step atan appropriate time), and such time can be set based for example on arelationship between an average height growth rate and a substrate'stemperature and a shape and average interparticle distance ofmetal-based particles of a metal-based particle assembly obtained, aspreviously obtained through an experiment. Alternatively a time elapsingbefore a thin film of a metal-based material grown on a substrateexhibits conduction (that is, a time allowing the thin film to be acontinuous film rather than a metal-based particle assembly film) maypreviously be obtained through an experiment and the particle growthstep may be stopped before that time is reached.

The metal-based particles are grown on a substrate preferably having assmooth a surface as possible and, inter alia, more preferably a surfacethat is smooth at the atomic level. When the substrate has a smoothersurface, thermal energy received from the substrate helps a growingmetal-based particle to merge with a surrounding, adjacent metal-basedparticle and thus grow, and thus there is a tendency to facilitateproviding a film formed of metal-based particles of a larger size.

The substrate on which the metal-based particles are grown can exactlybe used as a substrate of the quantum dot light-emitting device. Thatis, a substrate having a metal-based particle assembly layer layered orcarried thereon, as produced in the above method, (i.e., a metal-basedparticle assembly layer-layered substrate) can be used as a constituentmember of the quantum dot light-emitting device.

<Configuration of Quantum Dot Light-Emitting Device>

The quantum dot light-emitting device of the present invention at leastincludes: a light-emitting layer containing a quantum dot luminescentmaterial and a metal-based particle assembly layer described above.According to the quantum dot light-emitting device of the presentinvention, even when a quantum dot luminescent material exhibiting arelatively low quantum efficiency is used, the metal-based particleassembly layer mentioned above is provided in aim for enhancement of aphotoluminescence quantum efficiency, thus high luminous efficiency canbe exhibited. The quantum dot light-emitting device of the presentinvention may employ a configuration similar to that of a conventionallyknown quantum dot light-emitting device except that the metal-basedparticle assembly layer mentioned above is included in the device.

FIG. 1 is a schematic cross section view of an example for a quantum dotlight-emitting device of the present invention. The quantum dotlight-emitting device shown in FIG. 1 includes: a light-emitting layer50 containing a quantum dot luminescent material 55; and a metal-basedparticle assembly layer that is a layer (or film) consisting of aparticle assembly disposed in the quantum dot light-emitting device andformed of 30 or more metal-based particles 20 mutually separated anddisposed in two dimensions. As shown in FIG. 1, the quantum dotlight-emitting device of the present invention, as well as a typicalquantum dot light-emitting device, can be such constituent layers asdescribed above that are layered on a substrate 10.

In the quantum dot light-emitting device shown in FIG. 1, light-emittinglayer 50 emits light by irradiating light having a prescribed wavelengthcapable of exciting light-emitting layer 50. Further, other than theexcitation of the light-emitting layer by the method of irradiatinglight, as well as the quantum dot light-emitting device shown in FIG. 2described below, the emission of light can be performed also byproviding electrode layers (and function layers formed as needed) onupper and lower sides of the light-emitting layer and allowing emissionof light with electric driving, more specifically, by hole injectionfrom a positive electrode, and electron injection from a negativeelectrode and accordingly generating exciter in the light-emittinglayer.

According to the quantum dot light-emitting device with the electricdriving mentioned above, not only the enhancement of thephotoluminescence quantum efficiency but also improvements in lightextraction efficiency can be achieved, thereby exhibiting high luminousefficiency.

FIG. 2 is a schematic cross section view of another example for aquantum dot light-emitting device of the present invention, and shows astructure example of a quantum dot light-emitting device with electricdriving as one example of a quantum dot light-emitting device. Thequantum dot light-emitting device shown in FIG. 2 includes: a pair of afirst electrode layer 40 (e.g. an anode) and a second electrode layer 60(e.g. a cathode); a light-emitting layer 50 disposed between firstelectrode layer 40 and second electrode layer 60, and containing aquantum dot luminescent material 55; and a metal-based particle assemblylayer that is a layer (or film) consisting of a particle assemblydisposed in the quantum dot light-emitting device and formed of 30 ormore metal-based particles 20 mutually separated and disposed in twodimensions. As shown in FIG. 2, as well as a typical quantum dotlight-emitting device, the quantum dot light-emitting device can be suchconstituent layers as described above that are layered on a substrate10.

Not limited to the example of FIG. 2, while the metal-based particleassembly layer can be disposed at any position in the quantum dotlight-emitting device, the layer is preferably disposed closer to alight extraction face than the light-emitting layer 50, more preferablyin a vicinity of the light extraction face. As has been set forth above,the present invention allows a metal-based particle assembly layer topresent plasmon resonance to have an effect over a significantlyextended range, and accordingly allows the metal-based particle assemblylayer to be disposed at such a position in a vicinity of the lightextraction face apart from light-emitting layer 50 while ensuring a highemission enhancement effect. The metal-based particle assembly layerdisposed closer to the light extraction face allows more improved lightextraction efficiency and hence more improved luminous efficiency.Disposing the metal-based particle assembly layer in the quantum dotlight-emitting device is advantageous in shortening the fluorescencelifetime of the light-emitting layer receiving the plasmon enhance, andshortening the time during which the emission body is in an excitedstate, thereby suppressing the deterioration of the emission body.

For example, the quantum dot light-emitting device with electric drivingin one preferable configuration includes substrate 10, a metal-basedparticle assembly layer, first electrode layer 40, light-emitting layer50, and second electrode layer 60 in this order, as shown in FIG. 2.When such a configuration has substrate 10 that is a light-transmittablesubstrate (preferably, an optically transparent substrate), theconfiguration allows a light extraction face to be provided at a side ofsubstrate 10 opposite to a side thereof provided with the metal-basedparticle assembly layer (e.g., a back surface of substrate 10 (a surfaceof the substrate opposite to a side thereof provided with themetal-based particle assembly layer) can serve as the light extractionface), and thus allows a configuration with the metal-based particleassembly layer disposed in a vicinity of the light extraction face.

In the quantum dot light-emitting device shown in FIG. 2, themetal-based particle assembly layer can be layered directly on (orcarried directly on) substrate 10, and the metal-based particle assemblylayer and substrate 10 thus stacked in layers can preferably beimplemented by the metal-based particle assembly layer-layered substratethat can be produced in the above described method.

While substrate 10 in the quantum dot light-emitting device of thepresent invention may be formed of any material including a materialconventionally employed in a substrate of a light-emitting device, it ispreferable that substrate 10 be a non-conductive substrate to ensurethat the metal-based particle assembly layer is non-conductiveespecially when the metal-based particle assembly layer is layereddirectly on substrate 10. The non-conductive substrate can be formed ofglass, a variety of inorganic insulating materials (SiO₂, ZrO₂, mica,and the like), and a variety of plastic materials. Particularly, when alight extraction face is provided on substrate 10, substrate 10 ispreferably light-transmittable, more preferably optically transparent.

As shown in FIGS. 1 and 2, the quantum dot light-emitting device of thepresent invention preferably further includes an insulating layer 30interposed between light-emitting layer 50 and metal-based particleassembly layer, more specifically, covering respective metal-basedparticles 20 constituting the metal-based particle assembly layer. Suchinsulating layer 30 can ensure that metal-based particle assembly layeris non-conductive (or the metal-based particles are non-conductivetherebetween) as described above, and insulating layer 30 also allowsthe metal-based particle assembly layer and an adjacent layer to beelectrically insulated from each other. While the quantum dotlight-emitting device has each constituent layer with a current passingtherethrough, the metal-based particle assembly layer with a currentpassing therethrough may result in failing to obtain a sufficientemission enhancement effect via plasmon resonance. Providing insulatinglayer 30 that caps the metal-based particle assembly layer allows ametal-based particle assembly layer and an adjacent layer to beelectrically insulated from each other and can thus prevent a currentfrom being injected into the metal-based particles that compose themetal-based particle assembly layer.

Insulating layer 30 is formed of any material that is not specificallyrestricted as long as having satisfactory insulation, and it can beformed for example of spin on glass (SOG; containing organic siloxanematerial for example) and in addition thereto SiO₂, Si₃N₄ or the like.While insulating layer 30 is of any thickness that is not restricted aslong as ensuring desired insulation, it is better that insulating layer30 is smaller in thickness in a range ensuring desired insulation as itis preferable that light-emitting layer 50 and the metal-based particleassembly layer be closer in distance, as will be described later.

Light-emitting layer 50, first electrode layer 40, and second electrodelayer 60 can be configured of material conventionally known in the fieldof art and may also have thickness that a quantum dot light-emittingdevice normally has.

A quantum dot luminescent material 55 contained in light-emitting layer50 may be of conventionally known, and can be for example nanoparticlesmade of a semiconductor material such as MgS, MgSe, MgTe, CaS, CaSe,CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe,CdTe, HgS, HgSe, HgTe, GaAs, GaN, GaP, InN, InGaP, InGaN, InAs, InP,InSb, Si, Ge or the like and each particle having a diameter ofapproximately from 1 to 20 nm, more preferably approximately from 2 to10 nm.

Quantum dot luminescent material 55 may have a single layer structuremade of a single semiconductor material, or a core-shell structure asshown in FIGS. 1 and 2 in which a surface of a core particle (core layer51) made of a single semiconductor material is covered with a coveringlayer (shell layer 52) made of a different semiconductor material. Inthe latter case, the semiconductor material composing shell layer 52normally exhibits larger bandgap energy than that of the semiconductormaterial composing core layer 51. Generally, the core-shell structurehas a higher quantum efficiency than the single layer structure.

Light-emitting layer 50 contains at least one quantum dot luminescentmaterial 55, and generally contains a plurality of quantum dotluminescent material 55. In light-emitting layer 50, plurality ofquantum dot luminescent material 55 may be aligned in a single film,multiple films, or a particle aggregate film (a plurality of quantumdots are aggregated to form layers). Further, light-emitting layer 50may be composed of quantum dot luminescent material 55 only, or maycontain other constituent material (for example, a conductive matrixpolymer material supporting quantum dot luminescent material 55). In thelatter case, quantum dot luminescent material 55 can be dispersed in thematrix polymer material.

The quantum dot light-emitting device may further include other layerssuch as a hole transport layer, an electron transport layer, and thelike which may be included in a conventional organic EL device,inorganic LED device, and an inorganic EL device.

While the light-emitting layer of the quantum dot light-emitting devicemay have a thickness of for example 10 nm or larger, further 20 nm orlarger, still further the thickness larger than that, the presentinvention includes a metal-based particle assembly layer presentingintense plasmon resonance and plasmon resonance having an effect over asignificantly extended range (or a plasmonic enhancement effect coveringthe range), and thus allows a light-emitting layer as a whole to provideenhanced emission and hence improved luminous efficiency, even when thelight-emitting layer has a large thickness.

The quantum dot light-emitting device of the present invention is notlimited in a distance between the light-emitting layer and themetal-based particle assembly layer (a distance from the light-emittinglayer side surface of the metal-based particle assembly layer to thelight-emitting layer) and thus can achieve effective enhancement viaplasmon resonance with the metal-based particle assembly layer disposedat a position for example 10 nm, further several tens nm (e.g., over 20nm, 30 nm, or 40 nm), still further several hundreds nm away from thelight-emitting layer as described above.

For example, even if the quantum dot light-emitting device of thepresent invention has the light-emitting layer and the metal-basedparticle assembly layer with a distance of 10 nm or larger, further 20nm or larger therebetween, it nonetheless allows the quantum dotluminescent material contained in the light-emitting layer to have aphotoluminescence quantum efficiency (i.e., the number of dischargedphotons divided by that of absorbed photons) of 1.5 times or larger,furthermore, 3 times or larger than that of a reference quantum dotlight-emitting device that does not have the metal-based particleassembly layer.

Note that there is a tendency that, for its nature, the plasmonicemission enhancement effect decreases as the distance betweenlight-emitting layer and the metal-based particle assembly layerincreases, so it is preferable that the distance be smaller. Thelight-emitting layer and the metal-based particle assembly layer have adistance therebetween preferably of 100 nm or smaller, more preferably20 nm or smaller, and still more preferably 10 nm or smaller.

Preferably the metal-based particle assembly layer has a plasmon peakwith a maximum wavelength matching or close to the emission wavelengthof the light-emitting layer. This allows plasmon resonance to contributeto a more effectively increased emission enhancement effect by theplasmon resonance. The maximum wavelength of the plasmon peak of themetal-based particle assembly layer is controllable by adjusting thelayer's constituent metal-based particles in metal type, averageparticle diameter, average height, aspect ratio, and/or averageinterparticle distance.

EXAMPLES

Hereinafter, examples will be described to more specifically describethe present invention, although the present invention is not limitedthereto.

Producing Metal-Based Particle Assembly Layer-Layered SubstrateProduction Example 1

A direct-current magnetron sputtering apparatus was used to grow silverparticles significantly slowly on a soda glass substrate to form a thinfilm of a metal-based particle assembly on the entire surface of thesubstrate to produce a metal-based particle assembly layer-layeredsubstrate under the following conditions.

gas used: argon;

pressure in chamber (sputtering-gas pressure): 10 Pa;

substrate-to-target distance: 100 mm;

sputtering power: 4W;

average particle diameter growth rate (average particlediameter/sputtering time): 0.9 nm/minute;

average height growth rate (=average deposition rate=averageheight/sputtering time): 0.25 nm/minute;

substrate's temperature: 300° C.; and

substrate's size and shape: a square with each side having a length of 5cm

FIG. 3 is SEM images of a metal-based particle assembly layer in theobtained metal-based particle assembly layer-layered substrate, asobserved from directly above. FIG. 3( a) is an image enlarged as scaled10000 times and FIG. 3( b) is an image enlarged as scaled 50000 times.FIG. 4 is an AFM image of the metal-based particle assembly layer in themetal-based particle assembly layer-layered substrate obtained. The AFMimage was obtained via “VN-8010” produced by KEYENCE CORPORATION (thisis also applied hereinafter). FIG. 4 shows an image having a size of 5μm×5 μm.

A calculation with reference to the FIG. 3 SEM images indicates that thepresent Production Example provided a metal-based particle assemblylayer configured of silver particles having an average particle diameterof 335 nm and an average interparticle distance of 16.7 mm, as based onthe definition indicated above. Furthermore, from the FIG. 4 AFM image,an average height of 96.2 nm was obtained. From these values the silverparticle's aspect ratio (average particle diameter/average height) wascalculated to be 3.48 and it can also be found from the obtained imagesthat the silver particles have an oblate shape. Furthermore, it can beseen from the SEM images that the metal-based particle assembly layer ofthis Production Example has approximately 6.25×10¹⁰ silver particles(approximately 25 particles/μm²).

Furthermore, the obtained metal-based particle assembly layer-layeredsubstrate had the metal-based particle assembly layer connected at asurface to a tester [multimeter “E2378A” produced by Hewlett PackardCo.] to confirm conduction, and it has been found to be non-conductive.

Production Example 2

An aqueous silver nanoparticle dispersion (produced by Mitsubishi PaperMills, Ltd., silver nanoparticle concentration: 25% by weight) wasdiluted with pure water to have a silver nanoparticle concentration of2% by weight. Then to the aqueous silver nanoparticle dispersion 1% byvolume of a surfactant was added and sufficiently agitate and thereafter to the obtained aqueous silver nanoparticle dispersion 80% byvolume of acetone was added and sufficiently agitated at ordinarytemperature to prepare a silver nanoparticle coating liquid.

Then, the silver nanoparticle coating liquid was applied withspin-coating at 1000 rpm on a 1 mm thick soda glass substrate having asurface wiped with acetone and thereafter the substrate was left as itis in the atmosphere for 1 minute and subsequently annealed in anelectric furnace of 550° C. for 40 seconds. A silver nanoparticle layerwas thus formed, and on the nanoparticles layer the silver nanoparticlecoating liquid was again applied with spin-coating at 1000 rpm andthereafter left as it is in the atmosphere for 1 minute and subsequentlyannealed in an electric furnace of 550° C. for 40 seconds to obtain ametal-based particle assembly layer-layered substrate.

FIG. 5 is SEM images of a metal-based particle assembly layer in theobtained metal-based particle assembly layer-layered substrate, asobserved from directly above. FIG. 5( a) is an image enlarged as scaled10000 times and FIG. 5( b) is an image enlarged as scaled 50000 times.FIG. 6 is an AFM image of the metal-based particle assembly layer in themetal-based particle assembly layer-layered substrate obtained. FIG. 6shows an image having a size of 5 μm×5 μm.

A calculation with reference to FIG. 5 SEM images indicates that thepresent Production Example provided a metal-based particle assemblylayer configured of silver particles having an average particle diameterof 293 nm and an average interparticle distance of 107.8 nm, as based onthe definition indicated above. Furthermore, from the FIG. 6 AFM image,an average height of 93.0 nm was obtained. From these values the silverparticle's aspect ratio (average particle diameter/average height) wascalculated to be 3.15 and it can also be found from the obtained imagesthat the silver particles have an oblate shape. Furthermore, it can beseen from the SEM images that the metal-based particle assembly layer ofthis Production Example has approximately 3.13×10¹⁰ silver particles(approximately 12.5 particles/μm²).

Furthermore, the obtained metal-based particle assembly layer-layeredsubstrate had the metal-based particle assembly layer connected at asurface to a tester [multimeter “E2378A” produced by Hewlett PackardCo.] to confirm conduction, and it has been found to be non-conductive.

Comparative Production Examples 1 and 2

The direct-current magnetron sputtering was done with a variedsputtering time to produce metal-based particle assembly layer-layeredsubstrate of Comparative Production Examples 1 and 2. The metal-basedparticle assembly layer-layered substrate of Comparative ProductionExample 1 had metal-based particles having approximately the same shape,aspect ratio, and average interparticle distance as Production Example 1except that the metal-based particles had an average height ofapproximately 10 nm, and the metal-based particle assembly layer-layeredsubstrate of Comparative Production Example 2 had metal-based particleshaving approximately the same shape, aspect ratio and averageinterparticle distance as Production Example 1 except that themetal-based particles had an average height of approximately 30 nm.

Comparative Production Example 3

An aqueous silver nanoparticle dispersion (produced by Mitsubishi PaperMills, Ltd., silver nanoparticle concentration: 25% by weight) wasdiluted with pure water to have a silver nanoparticle concentration of6% by weight. Then to the aqueous silver nanoparticle dispersion 1% byvolume of a surfactant was added and sufficiently agitated andthereafter to the obtained aqueous silver nanoparticle dispersion 80% byvolume of acetone was added and sufficiently agitated at ordinarytemperature to prepare a silver nanoparticle coating liquid.

Then, the silver nanoparticle coating liquid was applied withspin-coating at 1500 rpm on a 1 mm thick soda glass substrate having asquare shape with 5 cm sides and having a surface wiped with acetone andthereafter the substrate was left as it is in the atmosphere for 1minute and subsequently annealed in an electric furnace of 550° C. for 5minutes to obtain a metal-based particle assembly layer-layeredsubstrate.

FIG. 7 is an SEM image of a metal-based particle assembly layer in themetal-based particle assembly layer-layered substrate obtained in thisComparative Production Example 3, as observed from directly above, andis an image enlarged as scaled 10000 times. Further, FIG. 8 is an AFMimage of the metal-based particle assembly layer in the metal-basedparticle assembly layer-layered substrate obtained in this ComparativeProduction Example 3. FIG. 8 shows an image having a size of 5 μm×5 μm.

A calculation with reference to FIG. 7 SEM image indicates that thepresent Comparative Production Example 3 provided a metal-based particleassembly layer configured of silver particles having an average particlediameter of 278 nm and an average interparticle distance of 195.5 nm, asbased on the definition indicated above. Furthermore, from the FIG. 8AFM image, an average height of 99.5 nm was obtained. From these valuesthe silver particle's aspect ratio (average particle diameter/averageheight) was calculated to be 2.79 and it can also be found from theobtained images that the silver particles have an oblate shape.Furthermore, it can be seen from the SEM image that the metal-basedparticle assembly layer of this Comparative Production Example 3 hasapproximately 2.18×10¹⁰ silver particles (approximately 8.72particles/μm²).

[Measuring Absorption Spectrum of Metal-Based Particle AssemblyLayer-Layered Substrate]

FIG. 9 represents absorption spectra, as measured throughabsorptiometry, of the metal-based particle assembly layer-layeredsubstrate obtained in Production Example 1 and Comparative ProductionExamples 1 and 2. As indicated in a nonpatent document (K. Lance Kelly,et al., “The Optical Properties of Metal Nanoparticles: The Influence ofSize, Shape, and Dielectric Environment”, The Journal of PhysicalChemistry B, 2003, 107, 668), an oblate silver particle as produced inProduction Example 1 typically has a plasmon peak around 550 nm and 650nm for average particle diameters of 200 nm and 300 nm, respectively (asilver particle alone in either cases).

In contrast, it can be seen that Production Example 1's metal-basedparticle assembly layer-layered substrate, with its constituent silverparticles having an average particle diameter of approximately 300 nm(335 nm), nonetheless presents for a visible light region a maximumwavelength of a plasmon peak at a longest side in wavelength, and themaximum wavelength is around approximately 450 nm, or shifted to ashorter side in wavelength, as shown in FIG. 9. This phenomenon can bemanifested when the silver particles are large-size particles having theabove prescribed shape and also have the above prescribed interparticledistance and are disposed significantly closely, as provided inProduction Example 1. Such a phenomenon would not rationally beunderstandable without considering that the particles that are closelyadjacent allow their respective, internally caused plasmons to interactwith each other.

Furthermore, the plasmon peak's maximum wavelength also depends on themetal-based particles' average particle diameter. For example,Comparative Production Examples 1 and 2 have small average particlediameters, and accordingly have a plasmon peak at a side considerablylonger in wavelength than Production Example 1, with maximum wavelengthsof approximately 510 nm and approximately 470 nm, respectively.

Further, Production Example 1 shows for the visible light region amaximum wavelength of a plasmon peak at a longest side in wavelength,and an absorbance at the maximum wavelength is approximately 1.9, whichis significantly higher than Comparative Production Examples 1 and 2 andit can be seen therefrom that Production Example 1 provides ametal-based particle assembly layer presenting significantly intenseplasmon resonance.

FIG. 10 represents an absorption spectrum, as measured throughabsorptiometry, of a metal-based particle assembly layer-layeredsubstrate obtained in Production Example 2. It present for the visiblelight region a maximum wavelength of a plasmon peak at a longest side inwavelength, and the maximum wavelength was 488 nm.

Note that the FIGS. 9 and 10 absorption spectra are obtained as follows:a metal-based particle assembly layer-layered substrate is exposed tolight of the ultraviolet to visible light region incident on a backsurface thereof (i.e., a side opposite to the metal-based particleassembly layer) in a direction perpendicular to a substrate surface andintensity I of transmitted light omnidirectionally transmitted towardthe metal-based particle assembly layer is measured with an integratingsphere spectrophotometer. On the other hand, a substrate which does nothave a metal-based particle assembly film and has the same thickness andthe same material as the substrate of said metal-based particle assemblyfilm-layered substrate is exposed at a surface thereof to the sameincident light as above in a direction perpendicular to that surface andintensity I₀ of transmitted light omnidirectionally transmitted througha side opposite to the incident surface is measured with the integratingsphere spectrophotometer. The axis of ordinate represents absorbance,which is represented by the following expression:Absorbance=−log₁₀(I/I ₀)

[Producing Reference Metal-Based Particle Assembly and MeasuringAbsorption Spectrum]

A method shown in FIG. 11 was used to produce a substrate having areference metal-based particle assembly layered thereon. Initially,resist (ZEP520A produced by Nippon Zeon Co., Ltd.) was applied withspin-coating on an entire surface of a 5 cm long and 5 cm wide sodaglass substrate 100 (See FIG. 11( a)). Resist 400 had the thickness ofapproximately 120 nm. Then electron beam lithography was employed toprovide resist 400 with a circular opening 401 (See FIG. 11( b)).Circular opening 401 had a diameter of approximately 350 nm.Furthermore, adjacent circular openings 401 had a center-to-centerdistance of approximately 1500 nm.

Subsequently, resist 400 having circular opening 401 was subjected tovapor deposition to have a silver film 201 deposited thereon (see FIG.11 (c)). Silver film 201 had the thickness of approximately 100 nm.Finally, the substrate having silver film 201 was immersed in NMP(N-methyl-2-pyrrolidone produced by Tokyo Chemical Industry Co., Ltd.),and settled in an ultrasonic device for one minute at room temperatureto peel off resist 400 and silver film 201 deposited on resist 400,thereby obtaining a reference metal-based particle assemblylayer-layered substrate in which only the silver film 201 (silverparticles) in circular opening 401 was left and layered on soda glasssubstrate 100 (see FIG. 11( d)).

FIG. 12 is SEM images of a reference metal-based particle assembly layerin the obtained reference metal-based particle assembly layer-layeredsubstrate, as observed from directly above. FIG. 12( a) is an imageenlarged as scaled 20000 times and FIG. 12( b) is an image enlarged asscaled 50000 times. A calculation with reference to the FIG. 12 SEMimages indicates that the reference metal-based particle assembly layerwas configured of silver particles having a particle diameter of 333 nmand an average interparticle distance of 1264 nm, as based on thedefinition indicated above. Furthermore, from a separately obtained AFMimage, an average height of 105.9 nm was obtained. Furthermore, from theSEM images, it has been found that the reference metal-based particleassembly had approximately 62500 silver particles.

In accordance with the above described measurement method using amicroscope's objective lens (100 times), absorption spectrum measurementwas performed for the metal-based particle assembly layer-layeredsubstrate of Production Example 1. More specifically, with reference toFIG. 13, a metal-based particle assembly layer-layered substrate 500 hada substrate 501 exposed at a side thereof (a side opposite to ametal-based particle assembly layer 502) to light of a visible lightregion incident thereon in a direction perpendicular to a substratesurface. The transmitted light that was transmitted to a side ofmetal-based particle assembly layer 502 and reached an objective lens600 of 100 times was condensed by objective lens 600 and detected via aspectrophotometer 700 to obtain an absorption spectrum.

For spectrophotometer 700 was used “MCPD-3000”, a spectrophotometerproduced by Otsuka Electronics Co., Ltd. for an ultraviolet and visibleregion, and for objective lens 600 was used “BD Plan 100/0.80 ELWD”produced by Nikon. The result is shown in FIG. 14. A plasmon peak at alongest side in wavelength for the visible light region had a maximumwavelength similar to that shown in FIG. 9 absorption spectrum, i.e.,approximately 450 nm. In contrast, when absorption spectrum measurementwas performed for the reference metal-based particle assemblylayer-layered substrate also in accordance with the measurement methodusing the microscope's objective lens, it presented for the visiblelight region a maximum wavelength of a peak at a longest side inwavelength, and the maximum wavelength was 654 nm. Production Example1's metal-based particle assembly layer-layered substrate presents forthe visible light region a maximum wavelength of a peak at a longestside in wavelength, and the maximum wavelength is blue-shifted byapproximately 200 nm as compared with that of the reference metal-basedparticle assembly layer-layered substrate.

The metal-based particle assembly layer-layered substrate of ProductionExample 1 presented for the visible light region a maximum wavelength ofa peak at a longest side in wavelength, and an absorbance at the maximumwavelength was 1.744 (see FIG. 14), whereas the reference metal-basedparticle assembly layer-layered substrate presented an absorbance of0.033. As the metal-based particle assembly layer-layered substrate ofProduction Example 1 and the reference metal-based particle assemblylayer-layered substrate were examined to compare the absorbances at themaximum wavelengths of their peaks at a longest side in wavelength, theywere compared for the same number of metal-based particles, and to doso, an absorbance obtained from an absorption spectrum was divided by aparameter corresponding to the number of metal-based particles, i.e., acoverage of substrate's surface by the metal-based particles, tocalculate absorbance/coverage. The metal-based particle assemblylayer-layered substrate of Production Example 1 presentedabsorbance/coverage of 2.04 (coverage: 85.3%), whereas the referencemetal-based particle assembly layer-layered substrate presentedabsorbance/coverage of 0.84 (coverage: 3.9%).

FIGS. 15 and 16 show absorption spectra of the metal-based particleassembly layer-layered substrates obtained in Production Example 2 andComparative Production Example 3, as measured in a method using anobjective lens (100 times) of a microscope. The metal-based particleassembly layer-layered substrate obtained in Comparative ProductionExample 3 presented for the visible light region a maximum wavelength ofa peak at a longest side in wavelength, and the maximum wavelength was611 nm. This maximum wavelength is substantially the same as that of thereference metal-based particle assembly film-layered substratecorresponding to the metal-based particle assembly film-layeredsubstrate of Comparative Production Example 3, and the metal-basedparticle assembly film of Comparative Production Example 3 substantiallydoes not show a blue shift. The FIG. 16 absorption spectrum provides forthe visible light region a maximum wavelength of a peak at a longestside in wavelength, and an absorbance at the maximum wavelength is 0.444and the metal-based particles cover the substrate's surface at acoverage of 53.2%, and therefrom absorbance/coverage of 0.83 iscalculated. This absorbance/coverage is smaller than that of themetal-based particle assembly layer-layered substrate.

The metal-based particle assembly of Production Example 2 hasmetal-based particles having a larger average particle diameter thanthat of the metal-based particle assembly of Comparative ProductionExample 3, and accordingly it is rationally inferred from theMie-scattering theory that the metal-based particle assembly ofProduction Example 2 presents a plasmon peak appearing at a longer sidein wavelength than that of Comparative Production Example 3. In reality,however, the metal-based particle assembly of Production Example 2 haspresented a plasmon peak at a shorter side in wavelength than that ofProduction Example 3 by as much as 100 nm or larger. This rationallyindicates that the metal-based particle assembly of Production Example 2presents a plasmon peak with a maximum wavelength shifted toward ashorter side in wavelength than the reference metal-based particleassembly in a range of 30 to 500 nm.

Producing Quantum Dot Light-Emitting Device and Assessing EmissionEnhancement Example 1 and Comparative Example 1

An insulating layer was provided using a spin-on glass (SOG) on themetal-based particle assembly layer of the metal-based particle assemblylayer-layered substrate produced in Production Example 1 so as to coverrespective surfaces of metal-based particles. Thereafter, alight-emitting layer containing a quantum dot luminescent material isprovided on the insulating layer to obtain a quantum dot light-emittingdevice (Example 1). The intensity of light emission from the device uponphotoexcitation of the quantum dot light-emitting device was comparedwith a quantum dot light-emitting device (Comparative Example 1) havingsubstantially the same configuration as the quantum dot light-emittingdevice of the present example except that a metal-based particleassembly layer is not provided as a reference system, and improved lightintensity could be observed.

Example 2

Silver particles were grown under the same condition as ProductionExample 1 to form on a 0.5 mm thick soda glass substrate the metal-basedparticle assembly layer similar to that of Production Example 1. Themetal-based particle assembly layer had metal-based particles having thesame shape and average interparticle distance as Production Example 1except that the meta-based particles had an average height of 66.1 nm.After forming the metal-based particle assembly layer, immediately a SOGsolution was applied with spin-coating on the metal-based particleassembly layer to have an insulating layer having an average thicknessof 10 nm layered thereon. For the SOG solution was used “OCD T-7 5500T”,an organic SOG material produced by TOKYO OHKA KOGYO CO., LTD., whichwas then diluted with ethanol. The “average thickness” means averagethickness as provided on a metal-based particle assembly layer having anirregular surface, and it was measured as thickness provided when theSOG solution was directly applied to the soda glass substrate by spincoating (this is also applied to the following reference and comparativereference examples). When the average thickness has a relatively smallvalue, the metal-based particle assembly layer may have the insulatinglayer formed only in a trough and may not have its outermost surfaceentirely covered therewith.

Then the quantum dot “Lumidot CdS 400, core-type quantum dots” (which isthe name of product) sold by Sigma Aldrich Co. LLC was applied withspin-coating at 1000 rpm on to an uppermost surface of the metal-basedparticle assembly layer having the above described insulating layer toform a quantum dot light-emitting layer with a considerably small singleparticle film scale, so that a quantum dot light-emitting device wasobtained. Herein, the “quantum dot light-emitting layer with a singleparticle film scale” means that quantum dot luminescent material 55 isformed so as to configure a layer generally with each single particle oninsulating layer 30, as shown in the schematic cross section view ofFIG. 17( b).

Example 3

A quantum dot light-emitting device was produced similarly as done inExample 2 except that the insulating layer had an average thickness of30 nm.

Example 4

A quantum dot light-emitting device was produced similarly as done inExample 2 except that the insulating layer had an average thickness of80 nm.

Example 5

A quantum dot light-emitting device was produced similarly as done inExample 2 except that the insulating layer had an average thickness of150 nm.

Comparative Example 2

Silver particles were grown under the same condition as ComparativeProduction Example 3 to provide on a 0.5 mm thick soda glass substratethe metal-based particle assembly layer described in ComparativeProduction Example 3. Thereafter immediately the same SOG solution asthat used in Example 2 was applied on the metal-based particle assemblylayer to have an insulating layer having an average thickness of 10 nmlayered thereon. Thereafter, similarly to Example 2, a quantum dotlight-emitting layer was formed on an uppermost surface of themetal-based particle assembly layer having the above describedinsulating layer to obtain a quantum dot light-emitting device.

Comparative Example 3

A quantum dot light-emitting device was produced similarly as done inComparative Example 2 except that the insulating layer had an averagethickness of 30 nm.

Comparative Example 4

A quantum dot light-emitting device was produced similarly as done inComparative Example 2 except that the insulating layer had an averagethickness of 80 nm.

Comparative Example 5

A quantum dot light-emitting device was produced similarly as done inComparative Example 2 except that the insulating layer had an averagethickness of 150 nm.

Reference Example

The “Lumidot CdS 400, core-type quantum dots” (product name) as used inReference Example 2 was directly applied with spin coating at 1000 rpmto a 0.5 mm thick soda glass substrate to form a quantum dotlight-emitting layer having a considerably small single particle filmscale, so that a quantum dot light-emitting device was obtained.

Quantum dot light-emitting devices produced in Examples 2 to 5,Comparative Examples 2 to 5, and the Reference Example were assessed foremission enhancement in level, as follows. In other words, withreference to FIG. 17( a) showing a system employed to measure thequantum dot light-emitting devices' emission spectra and FIG. 17( b)showing a schematic cross section view of a quantum dot light-emittingdevice, a quantum dot light-emitting layer 55 side, formed with a singleparticle film scale, of a quantum dot light-emitting device 1 wasexposed to excitation light 3 from a direction perpendicular to asurface of quantum dot light-emitting layer 55 to cause quantum dotlight-emitting device 1 to emit light. For an excitation light source 4emitting excitation light 3 was used a UV-LED (UV-LED375-nano producedby SOUTH WALKER, excitation light wavelength: 375 nm). Radiated wasexcitation light 3 obtained by condensing the light emitted fromexcitation light source 4 through a lens 5. An emitted light 6 fromquantum dot light-emitting device 1 in a direction of 40 degreesrelative to the optical axis of excitation light 3 was condensed by alens 7 and then transmitted through a wavelength cut-off filter 8(SCF-505-44Y produced by SIGMA KOKI Co., LTD) to cut the light of thewavelength of the excitation light 3 and then detected via aspectrophotometer 9 (MCPD-3000 produced by Otsuka Electronics Co.,Ltd.). FIG. 17( b) is a schematic cross section view of quantum dotlight-emitting device 1 including on soda glass substrate 10 ametal-based particle assembly layer constituted of metal-based particles20, an insulating layer 30, and a quantum dot light-emitting layer 55provided in this order, as produced in examples and comparativeexamples.

From the spectra of the emissions detected, integrals were obtained forthe emission wavelength ranges. The respective integrals obtained fromeach emission spectrum of the quantum dot light-emitting devices ofExamples 2 to 5 and Comparative Examples 2 to 5 were divided by theintegral obtained from an emission spectrum of the quantum dotlight-emitting device of Reference Example to obtain “emissionenhancement magnification” as represented in FIG. 18 along the axis ofordinate.

REFERENCE SIGNS LIST

1 quantum dot light-emitting device; 3 excitation light; 4 excitationlight source; 5, 7 lens; 6 emitted light from quantum dot light-emittingdevice; 8 wavelength cut-off filter; 9 spectrophotometer; 10 substrate;20 metal-based particle; 30 insulating layer; 40 first electrode layer;50 light-emitting layer; 51 core layer; 52 shell layer; 55 quantum dotluminescent material (quantum dot light-emitting layer); 60 secondelectrode layer; 100 soda glass substrate; 201 silver film; 400 resist;401 circular opening; 500 metal-based particle assembly layer-layeredsubstrate; 501 substrate; 502 metal-based particle assembly layer; 600objective lens; 700 spectrophotometer.

The invention claimed is:
 1. A quantum dot light-emitting devicecomprising: a light-emitting layer containing a quantum dot luminescentmaterial; and a metal-based particle assembly layer being a layerconsisting of a particle assembly including 30 or more metal-basedparticles separated from each other and disposed in two-dimensions, saidmetal-based particles having an average particle diameter in a range offrom 200 to 1600 nm, an average height in a range of from 55 to 500 nm,and an aspect ratio, as defined by a ratio of said average particlediameter to said average height, in a range of from 1 to 8, wherein saidmetal-based particles that compose said metal-based particle assemblylayer are disposed such that an average distance between adjacentmetal-based particles may be in a range of from 1 to 150 nm.
 2. Thequantum dot light-emitting device according to claim 1, wherein saidmetal-based particles that compose said metal-based particle assemblylayer are oblate particles with said aspect ratio of more than
 1. 3. Thequantum dot light-emitting device according to claim 1, wherein saidmetal-based particles that compose said metal-based particle assemblylayer are made of silver.
 4. The quantum dot light-emitting deviceaccording to claim 1, wherein said metal-based particles that composesaid metal-based particle assembly layer are non-conductive betweenadjacent metal-based particles.
 5. The quantum dot light-emitting deviceaccording to claim 1, wherein said metal-based particle assembly layerhas in an absorption spectrum for a visible light region a maximumwavelength of a peak at a longest side in wavelength, and the maximumwavelength is in a range of from 350 to 550 nm.
 6. The quantum dotlight-emitting device according to claim 1, wherein said metal-basedparticle assembly layer has in an absorption spectrum for a visiblelight region a maximum wavelength of a peak at a longest side inwavelength, and an absorbance at the maximum wavelength is at least 1.7. The quantum dot light-emitting device according to claim 1, wherein adistance between a light-emitting layer side surface of said metal-basedparticle assembly layer and said light-emitting layer is at least 10 nm.8. The quantum dot light-emitting device according to claim 1, wherein adistance between a light-emitting layer side surface of said metal-basedparticle assembly layer and said light-emitting layer is at least 10 nm,and said quantum dot luminescent material contained in saidlight-emitting layer has a photoluminescence quantum efficiency of 1.5times or larger than that of a reference quantum dot light-emittingdevice that does not have said metal-based particle assembly layer. 9.The quantum dot light-emitting device according to claim 1, furthercomprising an insulating layer interposed between said light-emittinglayer and said metal-based particle assembly layer.
 10. The quantum dotlight-emitting device according to claim 9, wherein said insulatinglayer is formed so as to cover a surface of each metal-based particlethat composes said metal-based particle assembly layer.
 11. A quantumdot light-emitting device comprising: a light-emitting layer containinga quantum dot luminescent material; and a metal-based particle assemblylayer being a layer consisting of a particle assembly including 30 ormore metal-based particles separated from each other and disposed in twodimensions, said metal-based particles having an average particlediameter in a range of from 200 to 1600 nm, an average height in a rangeof from 55 to 500 nm, and an aspect ratio, as defined by a ratio of saidaverage particle diameter to said average height, in a range of from 1to 8, wherein said metal-based particle assembly layer has in anabsorption spectrum for a visible light region a maximum wavelength of apeak at a longest side in wavelength, and the maximum wavelength shiftstoward a shorter side in wavelength in a range of from 30 to 500 nm ascompared with that of a reference metal-based particle assembly in whichmetal-based particles having a particle diameter equal to said averageparticle diameter and a height equal to said average height and made ofthe same material are disposed such that each distance between adjacentmetal-based particles may be in a range of from 1 to 2 μm.
 12. Thequantum dot light-emitting device according to claim 11, wherein saidmetal-based particles that compose said metal-based particle assemblylayer are oblate particles with said aspect ratio of more than
 1. 13.The quantum dot light-emitting device according to claim 11, whereinsaid metal-based particles that compose said metal-based particleassembly layer are made of silver.
 14. The quantum dot light-emittingdevice according to claim 11, wherein said metal-based particles thatcompose said metal-based particle assembly layer are non-conductivebetween adjacent metal-based particles.
 15. The quantum dotlight-emitting device according to claim 11, wherein said metal-basedparticle assembly layer has in an absorption spectrum for a visiblelight region a maximum wavelength of a peak at a longest side inwavelength, and the maximum wavelength is in a range of from 350 to 550nm.
 16. The quantum dot light-emitting device according to claim 11,wherein said metal-based particle assembly layer has in an absorptionspectrum for a visible light region a maximum wavelength of a peak at alongest side in wavelength, and an absorbance at the maximum wavelengthis at least
 1. 17. The quantum dot light-emitting device according toclaim 11, wherein a distance between a light-emitting layer side surfaceof said metal-based particle assembly layer and said light-emittinglayer is at least 10 nm.
 18. The quantum dot light-emitting deviceaccording to claim 11, wherein a distance between a light-emitting layerside surface of said metal-based particle assembly layer and saidlight-emitting layer is at least 10 nm, and said quantum dot luminescentmaterial contained in said light-emitting layer has a photoluminescencequantum efficiency of 1.5 times or larger than that of a referencequantum dot light-emitting device that does not have said metal-basedparticle assembly layer.
 19. The quantum dot light-emitting deviceaccording to claim 11, further comprising an insulating layer interposedbetween said light-emitting layer and said metal-based particle assemblylayer.
 20. The quantum dot light-emitting device according to claim 19,wherein said insulating layer is formed so as to cover a surface of eachmetal-based particle that composes said metal-based particle assemblylayer.
 21. A quantum dot light-emitting device comprising: alight-emitting layer containing a quantum dot luminescent material; anda metal-based particle assembly layer being a layer consisting of aparticle assembly including 30 or more metal-based particles separatedfrom each other and disposed in two dimensions, said metal-basedparticles having an average particle diameter in a range of from 200 to1600 nm, an average height in a range of from 55 to 500 nm, and anaspect ratio, as defined by a ratio of said average particle diameter tosaid average height, in a range of from 1 to 8, wherein said metal-basedparticle assembly layer has in an absorption spectrum for a visiblelight region a maximum wavelength of a peak at a longest side inwavelength, and an absorbance at the maximum wavelength is higher ascompared with that of a reference metal-based particle assembly in whichmetal-based particles having a particle diameter equal to said averageparticle diameter and a height equal to said average height and made ofthe same material are disposed such that each distance between adjacentmetal-based particles may be in a range of from 1 to 2 μm, on thepremise that the numbers of the metal-based particles are the same. 22.The quantum dot light-emitting device according to claim 21, whereinsaid metal-based particles that compose said metal-based particleassembly layer are oblate particles with said aspect ratio of morethan
 1. 23. The quantum dot light-emitting device according to claim 21,wherein said metal-based particles that compose said metal-basedparticle assembly layer are made of silver.
 24. The quantum dotlight-emitting device according to claim 21, wherein said metal-basedparticles that compose said metal-based particle assembly layer arenon-conductive between adjacent metal-based particles.
 25. The quantumdot light-emitting device according to claim 21, wherein saidmetal-based particle assembly layer has in an absorption spectrum for avisible light region a maximum wavelength of a peak at a longest side inwavelength, and the maximum wavelength is in a range of from 350 to 550nm.
 26. The quantum dot light-emitting device according to claim 21,wherein said metal-based particle assembly layer has in an absorptionspectrum for a visible light region a maximum wavelength of a peak at alongest side in wavelength, and an absorbance at the maximum wavelengthis at least
 1. 27. The quantum dot light-emitting device according toclaim 21, wherein a distance between a light-emitting layer side surfaceof said metal-based particle assembly layer and said light-emittinglayer is at least 10 nm.
 28. The quantum dot light-emitting deviceaccording to claim 21, wherein a distance between a light-emitting layerside surface of said metal-based particle assembly layer and saidlight-emitting layer is at least 10 nm, and said quantum dot luminescentmaterial contained in said light-emitting layer has a photoluminescencequantum efficiency of 1.5 times or larger than that of a referencequantum dot light-emitting device that does not have said metal-basedparticle assembly layer.
 29. The quantum dot light-emitting deviceaccording to claim 21, further comprising an insulating layer interposedbetween said light-emitting layer and said metal-based particle assemblylayer.
 30. The quantum dot light-emitting device according to claim 29,wherein said insulating layer is formed so as to cover a surface of eachmetal-based particle that composes said metal-based particle assemblylayer.